I think he conveyed some relatively information in those memories somewhere, but yeah, the important role is earlier, when he gives Harry the sword (though doesn't make his identity known, obviously). Snape's social skills as a teenager aren't all that of course, and he has his prejudices about mixed blood, precisely because he is one himself (much like Voldemort that way), but I think Snape as Lily knows him in school is probably the closest to the "real" Snape that we see in the series. He doesn't have to be a saint or even a particularly kind person to be on the good side and risk life and limb for the cause the way he does.

To some extent, that was required to keep up his cover, particularly when Voldemort came back in the picture. No doubt, some of it was just him being an ass - like I said, nobody said he was a particularly sympathetic character. And he certainly did sacrifice for it - it wasn't much of a sacrifice while Voldemort wasn't there, but after that, he risked his life constantly and had to walk a very fine line between helping the Order and keeping up his cover. And then of course had to kill Dumbledore - on Dumbledore's request because of the curse and all, but still, can't have been an easy thing for him to do, as is shown quite clearly in Snape's short talk with Harry later in that book, one of the very few times in the series that he actually loses his temper and lets some of his emotions show.

Again with the fine line between helping and keeping up his cover. The situation at Hogwarts doesn't get much attention in the final book obviously, but to me it's clear that Snape tries to shield the students as much as he can without risking his cover. His eventual death isn't very glorious or noble, but then it was never going to be - double agents rarely get to die noble deaths, that doesn't make them less important.

I agree with all this, it was excellently done, and in a way kind of necessary to make Harry his own man, and the actual hero responsible for defeating Voldemort, instead of merely Dumbledore's instrument. But Dumbledore being wrong about Snape would have meant that an 11-year-old's initial judgement of a man was more accurate than the great wizard Dumbledore's, and that Dumbledore had gambled everything for all those years on a pawn he shouldn't have trusted. That's more than just proving Dumbledore can make mistakes like anybody else, it makes him look a naive fool.

I thought his indirect responsibility for his sister's death was a bad enough flaw, myself... But no, thematically, I still think Snape had to be good - or, well, "good" is relative, the revelations of book seven don't exonerate him from being an ass and picking on vulnerable students, but you get what I mean.