Question: > 
> doomguy (protagonist of a video game) utters: **There's nothing wrong with you that I can't fix…with my hands!**
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To understand well with a context please check out this: [Doom Comic, Volume 1, No. 1, Page 1](https://www.doomworld.com/10years/doomcomic/comic.php?page=1) (warning: gore).
Can you tell me why he said this? Is that a idiom or what?

Answer: 
It's a variation on an idiom, or possibly an amalgamation of two to create a veiled threat. "*Nothing that cannot be fixed*" is a fairly common idiom, meaning that something which has been broken is easily repairable. It is normally a positive, reassuring thing to say.

To do anything "*by hand*", or "*with my (bare) hands*" means to do something yourself, manually, without the benefit of automation and perhaps with the bare minimum of tools.

What I understand is being said is that there is something 'wrong' with Doomguy's foe. Saying there is something wrong with a person can, in context, mean they are sick, evil, twisted etc. The suggestion that he will 'fix' this is a euphemism - he is actually going to attack him. And it would seem he is going to use his fists, hence 'with my hands'.
**"How do you feel?"** Ringo Star replied calmly **"With ma hands!"** ;)

This context is different. It is a verbal threat to assert physical constraints to fix perceived unwanted behaviors as opposed to some medical or mental issue. It could get ugly if he declines assistance. But then this is a macho super-aggressive comic strip character.