Anyone who knows about laws?

The charge will be some form of assault, whatever it's called where you are. I happen to be British so I know that, for example, in Scotland it will be assault, while in England or Wales it will be common assault, assault occasioning actual bodily harm, or assault occasioning grievous bodily harm, depending on how much damage you caused. Brain damage would undoubtedly be GBH. There is another little wrinkle with GBH in that unlike the other two, it doesn't have to be intentional. There is an offence of GBH without intent because Parliament has decided that causing serious injury should be punished whether it is intentional or not.

I see that you're American so I just said that to illustrate the issues. You would have to look up the law of whichever state you did this in to see what possible kinds of assault there are and where this would fit.

If he dies, then because it was unintentional, this would be manslaughter rather than murder. Mental state might matter for what the charge is. To take English law again, there is a defence to a charge of murder that you were criminally insane, and in that case you could be found guilty of manslaughter due to diminished responsibility instead.

Yes your intentions matter. Your intention was to punch him. Now it caused brain damage, which wasn't your intention, so you aren't looking at attempted murder or anything. Your intention WAS to punch him though so you are being charged with assault/battery depending on local laws(possibly both). If you want to argue your intention was to shut him up well then you're likely to just annoy the Judge/Jury and get yourself in more trouble. I mean not so much saying that was what started it, but if you act like you did nothing wrong because that was your intention then yeah you'll get in trouble. If his injuries directly lead to the death then you'll be charged with manslaughter.

It's going to depend on the state.

So you only intended to cause bodily harm, but instead caused great bodily harm? According to Wisconsin law, "Whoever causes great bodily harm to another by an act done with intent to cause bodily harm to that person or another is guilty of a Class H felony". (It would be a class E felony if you had intended to cause the great bodily harm.)

If the person dies, this may apply: "Whoever recklessly causes the death of another human being is guilty of a Class D felony." Whether you were being reckless is going to depend on the exact circumstances.

The penalties are:

For a Class D felony, a fine not to exceed $100,000 or imprisonment not to exceed 25 years, or both.

For a Class H felony, a fine not to exceed $10,000 or imprisonment not to exceed 6 years, or both.

What everyone else neglected to tell you is that a person you left in a disabled state can and will find a lawyer to sue you for what they will need to pay in extra help and medical costs for the rest of their life. It doesn't matter if you don't have any money now. They will garnish your wages, and everything else you ever see come your way, and use it to help support the person whose life you ruined.

If he saw the punch coming that is assault.

You hitting him is battery.

if he dies it is involuntary manslaughter.

If they can prove you intended to kill him, it is murder.

Battery. If he dies, manslaughter, involuntary or otherwise.

Man slaughter