Quan failed him; hence the vendetta.
Before I go further, everyone should know I do not support Dorner per se, but a person is innocent until proven guilty, and police departments are made up of men and women who can do evil things, too.
How do you know if Dorner wasn't framed for the murders? It makes more sense that someone in the LAPD would have a vendetta against Quan.
If Evans was indeed a lesbian, I can easily see a Gay Squad taking vengeance against Quan and Dorner.