Yes. But note that I did say complicit. Vader happily played along with Tarkin's little decision and made no objections when there were rational and strategic ones to be made. There were at least 1 billion people on that planet. So even if you only consider him 1/10,000 responsible (I consider it more, given that Vader was one of the most powerful people on that station and he is shown frequently conversing with Tarkin in private), that's still 100,000 innocent civilians men, women, and children. For me, as horrifying as the slaughter of the younglings was, I can still see some kind of thoroughly messed-up logic behind it. Not so with Alderaan. Given that, I don't think the murder of younglings makes Vader "too evil." If anything, my tipping point is Alderaan.