Humans are animals and various animal species act in mutual aid to an extreme degree, including humans(especially prior to the agricultural revolution).
Aside from that part I totally agree with you. Sith are generally forced into a position where they must do some evil things or be destroyed, but that does not necessarily make all of them evil. I think it is fair, however, to characterize Sith as being evil in general(at least in the popular perception of what evil is).
This.