10 years in prison? what is with these japanese stories and never making bad guys suffer any real consequences? at least in the chinese ones the bad guys end up actually paying the price of their actions...
nonono... Know your Shounen tropes aimed at primary/middle school kids...
Heroes/Protags don't kill people. Bad Guys™ need to be Imprisoned first, consumed by pride and anger.
Then meet a hooded stranger offering them the Power of Revenge™.
Then they turn into a Monster and go after the MC. Who
then, and only then,
may kill them.
Because killing Humans is Bad™, but killing Monsters is UltrPlusGood™.
Although this one is so Sweetness-and-Light MC might even revert the transformation through the Power of Friendship™ or similar bullcrap..