I agree that the jail thing wouldn't bother me at all. All kids hear that sort of stuff from siblings, TV etc. Remember cowboys & indians?? Lol!
I think the stoning to death is a bit odd. I'd like to know how a child could've heard it enough to understand & repeat it in the same way, you know? Like they might say I'll stone you, but for them to say the 'to death' part on the end...
But then, some kids are waaaay more understanding & talkative than others, & they do say things like that just from hearing it once on the news etc.
I might have just said something like 'thats not fair', or not nice or something, but treated it like a kids game that it was probably meant to be. The same as I have done in the past when I've heard little ones say strange things. Even my DD has told DD2 she wanted to kill her etc. She had no idea what it meant at the time.