IMHO, the kindest thing we could do is kill them quickly and send them to Judgement before God - “hasten the Final Meeting” - and He can take it from there. His Eternal Punishment would absolutely dwarf anything we could do to them here on earth.
I prefer the wisdom and methodology os St. Gregory the Great:
Brimstone calls to mind the foul odors of the flesh, as Sacred Scripture itself confirms when it speaks of the rain of fire and brimstone poured by the Lord upon Sodom. He had decided to punish in it the crimes of the flesh, and the very type of punishment emphasized the shame of that crime, since brimstone exhales stench and fire burns.
It was, therefore, just that the sodomites, burning with perverse desires that originated from the foul odor of flesh, should perish at the same time by fire and brimstone so that through this just chastisement they might realize the evil perpetrated under the impulse of a perverse desire.