Well, he never fully turned to the Dark Side, as we saw in ROTJ. Or maybe he fully tunred, but was never really evil. I think that distinction is there.
The story arc is about redemption. You can't be redeemed if you weren't evil.
I think people make too much of the "Once you start down the Dark Path" thing that Yoda always says. . . it's obviously not the case, given what Vader does at the end of RotJ. Yoda has been wrong before. It looks like he was wrong about that too.
But it's pretty f'ing clear that Vader was evil. You don't go around sabering scads of "younglings", killing everyone you used to care about (or attempting to) and taking part in the destruction of whole planets (the scale of this is often overlooked since planets seem like cities in SW) unless you are E-V-I-L.
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Don't agree... The KIDS were evil in his eyes.