I'm lazy and i'm just gonna copy something I've already said before because more people need to see it.
Okay, first off; Kayle hasn't ever been just "Good angel that does justice because justice is good" She has had 3 majorish rewrites and in none of them is she just "good".
- In her first lore she is apart of a regime that demands full obedience where asking "why" is a death sentence. Slowly she becomes tired of doing mission apon mission on just faith she asks "Why not strike them harder and end this war now and forever" and gets sent to Valoran. Nothing to do with justice at all or being good either, it's completely different then any other lore.
- This is the one (technically 2) telling that give us what people think of Kayle. She's basically an alien from another world where a long fought war is taking place. She is said to be extremely powerful and extremely beautiful but she doesn't show her face because the war has destroyed her mentally and she also tries to reform those who do evil but most of the time has to strike them down because she sees them as beyong redemption. The next part was added in later where Morgana with the help of summoners and the League go to destroy her but Kayle then joins the League as well to stall them. Sounds good right? That's without Morgana's angle where it's explained that angels (Kayle's side) are tyrants who can't see the bigger picture (sound familiar?) and while Morgana isn't exactly pure, she isn't exactly evil as Kayle is the one who started the conflict.
- Same thing as above but without the whole league bit
TL:DR for above: Kayle has always been arguably not good because she sees herself as judge, jury, and executioner for any of those she sees beyond saving and she's supposed to mirror justice which isn't inherently good (if you steal food for your starving family are you evil?)
Second: We already have purely evil characters, they're from the Void and The Shadow Isles and we already have purely good characters because I doubt anyone is gonna say Nunu, Lux, Neeko, and Lulu.