The warp wasn't inherently evil, I thought, it's just the swirling manifestation of conscious and sentient thought. It wasn't so bad until the Eldar orgied themselves a deamon god into existence and then the other three came from humans, apparently, doing so much evil they spawned chaos gods too. It's been bad for so long that it's become self-reinforcing, but the God-Emperor had some kind of plan to reset it or something I dunno.

All the Chaos Gods have an inverse "good" aspect to their portfolios too, Tzeentch isn't just betrayal and evil plans but also Hope, Nurgle isn't just pestilence and death but also Love, Slaneesh isn't just lust and hedonism but Beauty and I think Khorne was supposed to have Courage or something. The more "good" parts could be cultivated but the galaxy has been broken for so long that it's hard to say if anything good could ever come of the warp and if they're not better off just having the Tyranids eat everything and wipe the slate clean.