For me, the biggest revealing thing about Arthur just not being a very good human being, besides his self-pitying tirade that Murray calls him out on, is the fact that all of his happiest and most giddy moments are after he's just brutally murdered someone. The first two dudebros on the subway you could argue were self-defense, but the third one he hunts him down and puts him down like a wounded animal. After being in shock for like, thirty seconds, he starts dancing, then goes marching more confidently than ever down the hall and bangs Zazie Beetz (in his fantasies, anyway). Later, he murders Randall like a total savage, and then gets all dressed up and goes dancing down the steps being the most carefree we've seen him the whole movie. He's all delighted with himself immediately after shooting Murray, and even more gleeful riding in the back of the police car watching the riots and the city burning. Causing death and destruction is the thing that finally makes him happy. He's a worse person than Walter White (IMO).