Scans_Daily

At this point in time it might make sense; The Joker had recently tried to murder Luthor in Outsiders, so no love lost there, and as Infinite Crisis went on to point out, Luthor's Society of Villains actively didn't want the Joker as a member as he's just too unreliable, and would kill or betray any or all of them on a whim if he thought it was funny.

There's a line in Terry Pratchett's "Hogfather", where Susan (Death's grand-daughter) confronts a fairly Joker-like sociopath

'I think I know you,....,' she said, as sweetly as she could for Banjo's sake. 'You're the mad kid they're all scared of, right?'

'The giggling excitable one even the bullies never touched because if they did he went insane and kicked and bit. The kid who didn't know the difference between chucking a stone at a cat and setting it on fire.'

To her delight he glared at her.

'Shut up,' he said.

'I bet no one wanted to play with you,' said Susan. 'Not the kid with no friends. Kids know about a mind like yours even if they don't know the right words for it-'