I think the Sith beliefs aren't inherently evil, but the Sith put them into practice in ways that are undoubtedly cruel. The Jedi, on the other hand, while their beliefs may seem heartless at first, ultimately strive towards a higher form of existence that transcends mortal vices and limitations. In short, both sides have a similar capacity to do harm, but the Sith do what they do for selfish reasons, while the Jedi attempt to grasp the bigger picture.