"We can't even execute someone like this?"
For maximum punishment rotting in jail is the way to go. Maybe to send a message execution would be better, but it would make him a martyr.
He won't rot in jail, he will have warm meals, television, exercise, time to read, time to write his memoirs, etc. Not a great life, but not rotting either.
Whether he is made a martyr is in the minds of those who wish to see it as such. Meanwhile, we have tombstones as reminders of what he was convicted of participating in (through an act of omission). The just penalty here is death. The death penalty serves the ends of justice; and those who already rail against us with lies can distort that to mean whatever they want it to. They will anyway.