Here's my two cents.
Dark Side is evil, literally evil. The teachings of the Sith, a path to the dark side, are what ya make of them. But, the actual "side" of the force that gives force users dark power is definitely evil. In some of SW's older religions the dark side was called "Bogan" (and light, Ashla... could be off on that spelling). It was considered a deity, with goals and ambition, mainly destroying Ashla. Things in game, like Sel-Makor, on Voss, and the various dark-side entities show that there is a willful corruption at work. The only time the force was given real ambiguity, as far as good/evil go, was in KOTOR 2 thanks to the ever deceitful Kreia. Other than that game, the force has always had two sides, one good, one evil.
Obviously, not all sith are evil. Think there's one in the Knight storyline, but his name escapes me. (starts with P, but that's all I remember )