It seems to me that the ruthless response to someone I consider (a) dangerous enough in principle to be worth worrying about and (b) currently powerless enough that I can dispose of them as I wish with impunity, is to kill rather than imprison them.

Which suggests that all prisons are ruthful.

(For the record: Not only am I not encouraging it, I actively assert that killing prisoners is a lousy idea. We should not kill prisoners. Especially not identifiable prisoners.)