I think death sentence is something that have been abused so many times in the past, that we should learn to avoid it, and if we want to be fair, even the worst monster should get the same treatment, as terrible as it could sound. Killing a genocide is getting into his game. I guess life sentence (in a real prison, without any kind of prerrogative) could be even worst than death to a man like him. ...not if he gets a chance to escape from prison [or a group tries to help him escape], or instigate violence in prison.
Murderers escape from prisons, and they also kill in prison.
Kevin