I think Snape had the potential to be a good person, though I don't think that his capacity to love has anything to do with that. But think on how different he may have turned out had he not been bullied by the marauders? How different he'd have been had Lily loved him the way he loved her?

Certainly your actions are what indicates whether you are 'good' or 'bad', but environment and upbringing have a fairly large influence on that. I realise that you can ask the 'what if' questions about any of the characters in the HP books, but it is interesting to do so with Snape because even from Harry's biased point of view, there are flickers of integrity and indicators that he might have been different had he been loved and nurtured himself.

So I'd say that the fact that he loved Lily doesn't make him a good person; but that his capacity to love, along with a more caring environment (where as a result his self-worth would have been higher) indicates that he could have been.