They make villains who’s goals make total sense and are something to root for, so to balance that they make the villains kill innocents just to make sure the audience doesn’t accidentally side with them.
Poison Ivy's explicitly stated goals are mass genocide, how is that something to root for. She's not advocating for moving to solar or nuclear and reducing emissions.
That's the point. You have a Villain who wants something good (stop the destruction of nature) and then pair it with something very evil (genocide). It's a very common trope in media. It leads to the "Hero" of the story being able to defend the status quo again and again without ever challenging it.
That is a can of worms about on par with a leftist asking "In what order should I read theory"? There a 1001 correct answers, and there is no exact timeline about how you can delve into the character.
I have a decent bit of superhero knowledge and I dive into it every now and then, but I will not claim that I am some absolute authority on this subject and that I can give you a definitive list.
I will however recommend "Batman: Year One" as it is a good place to start with the character and that is considered one of the best comics of all time.
For a Batman/Joker story I would recommend the "Killing Joke" series, but I will say that is is fairly mature in its content, as it really attempts to delve into the Joker's psychology as a tragic but evil character.
The Batman stories in the "Flashpoint" Series is fun as a side story as it follows a world where the Flash makes one change and alters all of history, and Batman ends up being Thomas Wayne (the dad) rather then Bruce Wayne (the son), and this is a much more aggressive and violent Batman, with a pretty shady moral code as Thomas goes full capitalist and is pretty evil.
Not begrudging your interests, but I think this post kind of summarizes why I have never been able to read any western comics.
I like really concrete beginning/end points for fictional media. I like to be able to say "This is book 1, I can start here. This is book 4, it's the last one". I also hate jumping into any series in the middle, even if it is absolutely designed for it. Anytime I have thought about trying to read any western comics (or warhammer on the same note) it just seems like a massive ambigious undertaking.