People are attracted to these characters because they are well written tortured souls not petty misanthropes. Joker suffers from untreated mental illness and an unrequited quest for acceptance that is relatable but something that most simply internalize and suffer with. Like Thanos there is an element of relatable humanity there but with which he reacts in the most horrible of ways.
Thanos seems almost doomed to exist. Someone or something keyed the universe to have an option to partially destruct. That this is possible via a specific programmed sequence of events implies that it was meant to happen at some point. Thanos feels the time is now - that survival requires this destruction and that he is called to enact it. Its intentionally unclear whether he is acting through evil or that his innate power calls upon him to bear a cross of necessity that serves a greater good through a horrible act.

Also it’s just fun because they are make-believe, they aren’t hurting anyone or actively encouraging anyone to do harm. It’s like being repulsed by someone playing violent videogames - such a superficial empty judgment to pass.