So... I've noticed lately that whenever people bandy about the whole "someone (preferably Batman) should totally kill the Joker!" thing, they always mention something along the lines of "he once killed a busload of children!" or "he once gassed an entire kindergarten class!" as justification.
The thing is... has canon actually confirmed any of this? In anything other than a throwaway line from Joker (who would say ANYTHING to get under someone's skin, whether it was true or not) to verify it? The closest I can think of was in The Dark Knight Returns, where he killed (dozens of?) boy scouts with poisoned cotton candy. But that 1.) was an Elseworlds, and 2.) featured him dying painfully shortly thereafter.