It's the Justice League - its in the hob description.
The problem is that most of the Monsters of the Magical side are really good at Hiding since they have been around since the Age of Gods and Heroes and know all the Tricks,
Batman is a totem for the Bat which does its best to keep most of that stuff away and what does get through (because GOTHAM) he can usally handle or get one of his few friend's like Giovanni to help him handle
The Zataras are Magical so can never avoid it and while they tell the league how bad things are, I'm pretty sure they don't take it seriously because they never really encounter it
Billy deals with this stuff alot in his city because of the Rock of Eternity and the fact that the city is a Nexus, but because of his Power the really Evil stuff Avoids him when possible.
Diana would know because of her education on how the World was during the previous Ages but like Billy because of her Power most of the Smart ones do the best to stay out of her way.
Aquaman deals with this stuff alot it's actually one of his Job as King to deal with them in the Oceans when his Armys and Mages can't. Which means that Aqualad definitely would know about some of the Dark stuff since ID this stuff would have been part of Mage courses at the Conservatory.
I don't have to get into Blood & John
The rest of the Leaguers only encounter the true Dark stuff (sacrifices Babys, Blood Farms, etc.... you know the True Evil stuff that Hellblazer deals with on the monthly) very rarely if at all so 99% of them don't really know or understand how Bad it is.
But if the Sidekicks start using Magic they will start running into this stuff as that is one of the Prices for Using Magic in DC, it's worse for Robin since he lives in Gotham where alot of that stuff happens alot.