Yeah, Arthur in the first "Joker" seems like too much of a good guy. He killed and murdered several people, including A late-night comedian (Robert De Niro reimagined "King of Comedy" early 80's character) by shooting him on live television, but his crimes and the path his life took, with all its difficulties, his past mental illness and current struggles with it, discovering his own mother was a psychopath who may/may not have had a relationship with a young Thomas Wayne, him working in a endless cycle of dead-end, demeaning jobs where he's constantly disrespected and walked over. His crimes or him going insane make more "sense" under those grim circumstances then the "real Joker" who is an unapologetic psychopath, presumably in the comics, its inferred the Joker was a feared, terrifying mob hitman that was called only to do certain, unspeakable crimes by the Falcone crime family and even then, his viciousness, level of sadism and cruelty and barbarity is so immense, it eventually turns even then off and then as the "Red Hood", well, we all know that little encounter went down between him and Batman at the Ace Chemical plants.
The Joker who becomes Batman's arch-nemesis is irredeemable, an unabashed anarchist who, unlike X-Men's Magneto, isn't meant to be understood. He's like Caligula on steroids.