Yeah everything you just said points exactly why he would turn into a villain LOL. Also, whats the fascination with people only wanting villains to be this all bad never good characters? That's boring, that's why people loved Arthas so much, he was a good guy, he did what he thought was good for his people and went mad over trying to defeat malganis. He isn't really 'all' evil.
Same thing with Illidan. From the start he was an enemy/good guy. We had common enemies, but the nightelves deemed him to be dangerous and a 'beast'. He went into outland to fight the burning legion and overthrow them (this is the story during tbc). We thought he was trying to gain an army and enslave the locals there so he had to be dealt with. What you're saying just doesn't make much sense for why you don't understand how this could happen. We screwed him over in wc3 time and time again, he helped us and we still treated him like a tool and warned him to basically just stay the fuck away. If you're upset with the story, it started in WC3 and that's what you should be upset about. TBC just continued on the path that these characters progressions were leading towards.
As for Kael, yeah it's the perfect story (just like literally every. single. villain. in. warcraft.), he was once on our side, got fucked over or driven mad, and allied with Illidan which at the time we thought was 'bad', and went to outland (with the burning legions command) to take over that world to defeat arthas. Which at the time, the humans/elves/the players character didn't know this was what he was going to do.
I'm just convinced you guys didn't pay much attention to the story in WC3 because it makes perfect sense.
Honestly, I would just watch videos like this if you are bored, it makes way more sense
https://www.youtube.com/watch?v=fEi0z3Bdd8M