Yeah, he was always a twisted soul. He was only ever a low-ranking serial killer when he wasn't a planeswalker. Now that he is he's a more effective threat, and even then only here because there's a clear pathway in the form of the sword.
It's not that I don't think he's a terrible person, it's that he seemed to be motivated more by personal amusement than any sort of philosophical commitment to "chaos". As far as I can tell, he's never even mentioned "chaos" before. It feels like they had a villain in mind and then just looked into their roster of planeswalkers to see whom they could most easily fit into the mold. And it's weird to see them trying to turn a Hannibal Lecter style small-scale villain into a comic book scale world-ending threat.
I'm not really a comic book guy, but isn't Joker kind of a mid-grade crook and occassional terrorist, largely restrained from more by his lack of superpowers? Other than one arc where he gains some weird absolute godlike powers? ("Emperor Joker" I think I've heard it called). Seems kind of apt.
I was referring more to the Dark Knight era recent Joker where he has some weird philosophical ramblings about wanting to cause chaos and take over the entire city or something. Which I guess is interesting because they kinda did the same thing there that WotC is doing here, and turned a small-scale crook into a human embodiment of "chaos" and an existential threat to society.