In general, chaos is seen as evil because they are emotion unbound by morality or law of any kind and way, to Slaanesh wanting to kiss someone, wanting them to be with you and wanting to rape and torture are the same thing because they come from the same root: to desire something, is just one have more bound because people concern for the well being of other and for morality reason, the chaos gods erase this(or demand you do so), so khrone concern with slaughter even with their own, slaaneshi are pleasure for pleasure sake and tzeetch plan and plot without end because there is not end.

Now with orks...they influence gods sure, but they are hard to be draw by them because they dont have angst, they have need and wants but they dont suffer if they are denied and more of it, they are somewhat animalistic.

At good example is the lizarman from fantasy, both are old ones weapons, both are both animalistic and borderline robotic and because of that, chaos have a hard time grapsing them.