my understanding of the Dark Side is that to become a master of the dark side of the Force—not just able to use it, but to control it and to bend it to your own will—becoming unquestionably evil is pretty much a requirement.
Does Vader fall under that?
Are you seriously asking if Vader is unquestionably evil?
Vader wasn’t unquestionably evil. He was pretty evil, but “there was still good in him.”