Yeah, I hate to spout the specific tenets of The Joker here, but I think pretty much anybody is capable of this sort of thing if you press the right set of buttons.
Maybe. But lots of fucking people have had messy divorces and lost their jobs.
I mean, all right, if you stuck me in Room 101 and tortured me, I'd probably be capable of anything too. But this falls pretty fucking short of that; in fact it's something pretty damned common.
The details of the plot are part of what makes it exceptional, too. If he'd JUST shot up his ex and his in-laws, that'd be abhorrent, but it wouldn't really be "WTF?" territory. A normal guy who snaps and kills people who he believes have wronged him is a common enough story. A guy who dresses up like Santa Claus, shoots a little girl in the face, and then burns the house down with a homemade firebomb goes into, yes, Joker territory.
(And incidentally, since you bring it up, the Joker was wrong, in both The Killing Joke AND The Dark Knight.)