Punishment is a tough decision.
Emotionally, I think fastening them to a rotating spit over hot coals and slowly moving more of them over the fire, starting with their feet is what they deserve. They will be burnt to death, sooner or later, the sooner the easier it will be for them. Let them die admist the pain of their charred body and smell of their own burnt flesh. But that is from their nature, not mine.
On the other hand, I cannot consider anyone who is capable of these things as human. Mad animals is the best I can give them...and we don't punish rabid dogs or enraged animals...we just put then down....as soon as possible.