Okay. You're a group of supervillains exiled for life on a distant planet. Among the very large group of villains is the Joker, who took the transition hard enough to be unconscious for awhile.
WHY DIDN'T THEY KILL HIM? Who would *want* the Joker around for any length of time? He doesn't work and play well with others, and it's not like this crowd is antimurder or anything, or is going to be troubled by killing a defenseless man. He's a total liability. Yeah, yeah, there was some hero worship, but...not everyone is stupid. If Deathstroke or Luthor had been in the group, you'd have one dead killer clown.
In fact, I'd think the folks who stranded them were kinda hoping for that.