Sure, most of the Necron race is braindead, but so what? Why should I feel bad for an entity that can feel no emotions in the first place? And the Necrons that still retain their emotions and personality seem like pretty stand up guys.
And Chaos isn't the evil made manifest, it's the emotions made manifest. That includes the good ones, love, joy, camaraderie, hope. Of course, Chaos shouldn't win, otherwise like anon said everything will die out, but being a Chaos cultist in and of itself isn't so bad.
And yeah, the Tyrannids are neither evil nor good. They're just an animal going about its basic functions, can hardly blame 'em for that. Not that I want them to win, but I wouldn't feel particularly bad if we're all eaten by them either.