So they say, but that's often the easy way to weasel yourself out of things. McGonagall for example also never got the only man she ever loved, just like Snape never got the only woman he ever loved, but McGonagall still *was* fair and just... Strict... terribly strict... yes... but just. I think I might have bonded with her well if I was a Hogwarts student (even though I feel the Sorting Hat would put me in Ravenclaw rather than Gryffindor, meaning that Flitwick would then be head of my house).
Harry could not help being the evidence Lily chose an other man, and he also couldn't help he looked so much like James, whom Snape loathed for reasons I can completely understand, since James *was* a swine... at least to Snape, since for James goes the same story... Yes James was a great man, as evidenced by his contributions in the battle against Voldemort and his will to sacrifice himself for those he loved, but has also been a terrible boy, and a first class bully.
The good does not erase the bad, but also not vice versa. I still can't like Snape as a person, although I can't deny that what he did and the key-role he played in Harry's battle against Voldemort (which he couldn't have won without Snape's help).
But I tell you what.... It *is* possible to love and hate the same person. I have both admiration as disgust towards Snape.