system
Given the following post, respond as if you're commenting on Reddit:
human
[My Hero Academia]I'm a teenager with a powerful Quirk, but the thing is, I can't control it completely yet. I'm eating at a restaurant when a villain busts through the door and begins killing people, but when I attack him, I accidentally instakill him. Brutally. What legal repercussions do I face?
gpt
MHA's society is very strict towards vigilantism.

When Midorya, Todoroki, and Iida fought Stain they were told in no uncertain terms that they were wrong to act. If they were given credit for Stain's capture both them and the pro heroes supervising them would have faced legal repercussions. This was against an admitted serial killer who intended to kill them. None of that mattered because they used their quirks without the proper permits.

When villains attacked the training camp with the intention of killing students, Eraser Head had to give them permission to defend themselves. Otherwise they would have faced similar repercussions.

The whole point of taking the provisional license exam was so the students could legally defend themselves next time they were attacked.

Clearly the law does not care if your life is in danger. It is illegal to use your quirk violently without a hero license. 

As for your case, you are a perfect example of why untrained civilians are not allowed to use their quirks on others. You knew when you used your quirk that you couldn't control it. If you had the proper training that villain wouldn't have died. They have to prosecute you, otherwise your act will inspire others to act as vigilantes. The government is terrified of losing control of its population. If everyone believes they have the right to use their quirk as they please it would lead to chaos. The authorities believe that faith in heroes is the only thing keeping society from falling apart. If no one had seen you defend yourself, things might be different. The police would probably sweep this under the rug, giving credit to whichever hero arrived first and helping you find a course that could teach you to control your power. But this was a crowded public place. People know what you did.

As for your punishment, it won't be severe. You most likely won't go to jail for this. You're a minor who was defending yourself. They'll probably force you into a hero course so you can receive proper training. Afterwards they'll prop you up as an example of how the system works. Without training you were a danger to yourself and others. Now you are a hero protecting the world from those that misuse their power.