I find it amusing how, in quite a few series like this (Nidome no Yuusha did it too), the only genuinely good person in the story is the villainous Demon Lord trying to take over the world and slaughter humanity.

Not even in a "better by comparison" way, they actually end up being the most genuinely morally upright characters, with everybody else, most of the main cast included, being such utter scum you agree with the Demon Lord's interest in conquering and genociding humanity, since they'll only cull the deserving people while leaving the genuinely innocent.

"people who write this stuff and people who enjoy it have serious problems man."

I agree with you that the series is shit, but maybe don't pass value judgement on the people who write it or read it. Certainly, plenty of fucked up people no doubt enjoy it, but most of the people I know who read Redo of Healer like it specifically because of how trashy and edgy it is. Like enjoying a bad horror movie.

As for the writer? They have written like 12 different Novel Series, plenty of which already have Manga Adaptations, and absolutely none of them are anywhere near as edgy as Healer, in fact most are pretty tame slice of life. Redo of Healer is a series he started writing specifically to write the edgiest thing imaginable.