I'm against the death penalty. I think a fitting punishment would be induced fractures of his cervical vertebrae to paralyze him from the neck down, then some surgery to render him blind, deaf and with no sense of taste or smell. Following this he should be administered anxiety-causing medicines for the rest of his life.
The darkness would feel quite lonely and might give him the chance to do some much needed introspection. It might even rehabilitate him.
It costs a lot of money better spent on the school system in the US.