1. The killer will always have the guilt feeling in him and his brother figure's screaming and squirming will haunting him. This will make his life miserable. This is what happens with all criminals who attack and slay people; they will remain under constant worries and fear from being chased by the ghosts of those who were killed by them.
In addition, they will be hunted down law or human rights activists, even a long time after the crime.
2. According to Islam, the biggest punishment is losing eternity and the killer being thrown into hell by God on the judgment day.