Tjyylli

Your sample dialogue sounds unnatural because it's on the nose. If you're not familiar with that term, it means, essentially, that there is no subtext. The characters say exactly what they think, feel, and mean; and they do it in a perfectly articulate manner. The reason on-the-nose dialogue is bad is because humans generally aren't articulate.

Sure, someone who is naturally charismatic or a highly practiced public speaker is capable of being articulate in this manner - but even then, they do much better when prepared than in the heat of the moment. Most people, who are neither trained speakers nor gifted with exceptional charisma, are going to have an even harder time expressing themselves clearly. Especially in a tense, emotionally-charged scene, people aren't taking the time to think about what they're saying or how best to clearly express it. In real life, people stutter, hesitate, cut themselves or each other off, talk around sensitive subjects, or refuse to talk altogether.

To assess whether dialogue sounds unnatural or cringy, examine how easily your characters are conveying their main points. If they're speaking with perfect clarity about exactly what they think or feel, your dialogue is most likely unnatural.

To apply this to your sample dialogue:

Natalia: You turned me into a monster, how could you do that to me?

She's addressing her concern exactly: that she's been turned into a monster. Instead, have her dance around it - talk about how she can't go out in public anymore, or even just have her be non-specifically horrified.

Robert: It was the only choice! You think we could have survived
otherwise? I made the call, because of that we're both alive. How
can't you see this!?

He immediately understands her concern, articulates his reasoning perfectly, and makes an immediate counter-argument. But if she's already hedging around the subject, he'll have to work harder to understand why she's upset - maybe even get it wrong at first.

Natalia: You made the choice without letting me decide my own fate. I
cannot ever forgive you for this!

Again, she's clearly explaining why she's upset about this. If this is a new situation, she may not even have figured out yet that the lack of choice is what's most upsetting about this. You could have them argue back and forth for a while longer while she slowly realizes that this is the crux of the issue.

Robert: I don't care! Do as you want. If you want to die, go ahead,
kill yourself. It's as simple as it gets. No matter what you tell me,
it's not going to do any good. What's done is done!

He's getting to say too much. If Natalia is truly as upset as she sounds, she'd have interrupted him by now.

Natalia: You pig!

Aside from the fact that this retort doesn't make much sense in context (usually calling someone a pig means they've been crudely sexist or otherwise gross, as opposed to morally or ethically faulty), it's too mild for the argument up til now. If she's really so mad about what he's done, she'd have some stronger words for him - or she'd nope right out of the conversation in fury.

Consider instead:

Natalia: What have you-- Oh my God. I'm--I'm-- What am I? What have
you done?!

Robert: Hey, wait, you're mad at me? What for? I made you stronger!
You're powerful now! You can save us!

Natalia: But I'm... I'm... This isn't right. I didn't want this. Why
did you do this to me?!

Robert: I wanted to survive! I wanted both of us to survive!

Natalia: I don't want to survive like this! If I had known surviving
meant becoming a monster, I never would've agreed!

Robert: I was trying to help! Do you want to die?

Natalia: I wanted the choice!

Obviously not perfect as I don't know the details behind the situation and am writing this off the cuff, but the point here is to add layers of subtext, confusion, and implication so that the conversation builds up to a climax as both characters slowly realize what the true issue is.

The key is to remember that humans almost never express themselves clearly and perfectly on the first try. If your characters are speaking articulately about the exact issue(s) at hand (even if they're being emotionally heated about it), your dialogue is likely on the nose. Add subtext, inferences, implications, misunderstandings, and other layers to give your dialogue the depth of real human speech.

Read it aloud. Flaws are often more apparent when heard - particularly in matters of flow and pacing.

Would Natalia say that? How would Robert answer or would he even bother?

Get inside your character’s head. What would you be feeling, thinking and eventually saying? Would you say anything at all?

Perhaps a glare of astonished hatred would serve and say more with silence than words. Have her think livid thoughts, feel the outrage and loss of control. Be her - then slip inside him and respond.

Would Robert just walk away, his task complete? Might he simply look at her with fond regret, seeing the alterations of which she is as yet unaware.

She woke - almost surprised to as the last thing she remembered was
getting hit, falling. Robert was there, had called for help. Wait -
that blaster hit should have killed her - was killing her. Had it?

She saw Robert, his back to her. He closed his surgical kit and
disposed of four syringes. Wait - red syringe meant bots. God, not
that. She clenched the side of the bed, not noticing the damage to the
rail.

Robert turned, seeing her glare. He noticed the damaged rail and knew
it was a complete success. It would take time for the bots to complete
their work and the prosthetic devices would work as well as her
natural arm had - maybe better. The eye was not a good match to her
natural colour, but she would be operational in a matter of days.

He should avoid her until she adapted to her new life, she would thank
him later. No choice, no time to waste. If she didn’t thank him, at
least she was alive to hate him.

Sadly I don't think there is an ISO standard test or something this can be run against, but like most parts of writing we can apply various tools to help us evaluate things.

Examples of some of the evaluation steps I run stuff through:
1. Does it fit with the character's other dialog, and does the pacing and tone match the scene? "I cannot..." while in a hurry might not flow as well as "I can't", unless someone is super formal by nature.

2. 
   

   Does it sound good?

   
   
   
   
   
   • Read it to yourself, have someone else read it to you, record and play it back, and ask yourself "Does this work?"
   
   
   • I personally find piping my work through text-to-speech software surprisingly handy for this.
   
   
   
   


3. 
   

   Does it get good feedback?

   
   
   
   
   
   • Beta readers are your friends [but sometimes you shouldn't use your friends for beta readers - Joining a writing circle or similar may be useful.] - Do other readers find it stands out in odd ways?
   
   
   
   


4. 
   

   Can better words be found?

   
   
   
   
   
   • If you're unsure about if you like a group of words, there is always the option to set the current ones aside and just rewrite the section for the sake of deciding if you like one better than the other.
   
   
   
   

Also keep in mind the point that at times much more can be said with silence than with a thousand words.

Tales handed down to us relate that whenever Ray Bradbury finished the first draft of anything, he would set it aside somewhere and not go back to it for an entire year.

Doing this gives you time to forget what you were thinking when you wrote the first draft, and so your brain is no longer smoothing over the rough spots.

Unless you are currently on a deadline, you can afford to do this. Finish the draft, shelve it, and move on to the next idea you've cooked up.

I won't add to the answers here which are bang on. Your example dialogue is just too telling, the characters aren't having a natural conversation, they're conveying information and that's why it sounds wrong. You need to convey their thoughts and feelings non-verbally and where you can't do that, add exposition.

Dialogue takes practice, you'll get better and better the more you write, so just keep redrafting and redrafting. But it isn't the place to convey information like some villain who's telling the hero his whole plan before he's about to kill him.

I watch a lot of movies. A well-written script can be a great tool for analysing dialogue because there's no room for exposition and you can see what is said directly, what can be read between the lines, and what is conveyed non-verbally. Crimson Tide is an excellent example. Watch Gene Hackman and Denzel Washington carefully and see how much they convey with a look, or say between the lines. You can watch the tension between the different schools of thought build up into a crescendo between these characters with scenes where they rarely say what they're actually thinking, e.g.:

CAPTAIN: Feels like the whole crew needs a kick in the ass.

XO HUNTER: Or a pat on the back, sir. I just witnessed a fight down in
crew's mess. No big deal, but... I think the men are... a little on
edge with all we're going through. Morale seems to be a bit low.

CAPTAIN: Well, you seem to have the pulse of the men.

XO HUNTER: Thank you, sir.

CAPTAIN ON THE 1MC: May I have your attention, please? Mr. Hunter has
brought it to my attention that morale may be a bit low... that you
may be a bit...

XO HUNTER: On edge, sir.

CAPTAIN: On edge {cruel smirk}. So I suggest this: Any crew member who feels he
can't handle this situation can leave the ship right now! Gentlemen,
we're at DEFCON three. War is imminent. This is the captain. That is
all.

XO HUNTER: Very inspiring, sir.

The other thing that could be helpful is to find some friends (good actors if you can find them) and either get them to act out your dialogue, or, even better, give them your scenario (he's just turned her into a monster, it's diabolical to her but it saved their lives) and see what dialogue they come up with on their own.

Either way, you have to hear it out loud, and you have to keep practicing, editing and rewriting.

Good luck!

I once saw a quote that went along the lines of:

If I know myself, I can act any role.

Focus on projecting yourself on these characters. Use your memories of relatable past events you've gone through to help. Convince yourself that you are in their shoes. You lived through everything they lived. Once you are them, how do you react to what just happened? If you got turned into a monster (whatever that means in detail), would you really react with the following?

You turned me into a monster, how could you do that to me?

I don't know what your idea of being turned into a monster is, but I think I would just scream and probably cry. It would take a while to start a conversation like that. In fact, instead of starting a conversation I would probably change to start shouting incomprehensibly at Robert and trying to kick his ass.

Who knows though, maybe your character has reasons to not act as horrified. Maybe she's been maimed before, maybe she has more important things to focus on, maybe she fears or admires Robert too much, I don't know.

In conclusion, I would stop thinking of these characters as people different from yourself and start thinking of them as yourself. Turn empathy to 11.

By the way, using your own memories to evoke genuine emotion to know how to act is part of what I believe is called "method acting". [1] If you do it effectively, take heed of the potential psychological effects. [2][3]

[1] https://en.wikipedia.org/wiki/Method_acting

[2] https://en.wikipedia.org/wiki/Method_acting#Psychological_effects

[3] https://en.wikipedia.org/wiki/Psychological_effects_of_method_acting