from DH and the literature, ignoring Chaos Marines, 40k just seems to lack decent villains.
descriptions start off interesting - brilliant guy trained in an academy of evil, or corrupted priest with a mysterious background, or highly talented, rich and degenerate noble.
But then they boil down to - seems to randomly cause trouble. Is psychotic and evil. Has no known goals. Just likes to be mean to people.
Are there any villains out there that aren't just psychotic? WIth a goal of bringing a loved one back to life, getting revenge on one particular person, or some other simple (but possibly megamaniacal) goal?