Would you forgive your BF if he left without saying good bye? And then returning almost 2 years later...?

If he was the love of my life, I'd want to hear him out and see what he has to say for himself. Most guys aren't very good about expressing their emotions and making themselves vulnerable that way, so they often pull away abruptly rather than having to try to explain themselves. You know he was dealing with major issues at the time -- maybe he just couldn't deal with all the emotion of a relationship, too. If you don't have the heart to understand that and at least hear him out, then just tell him that there's no possibility of reconciliation so he can move on with his life.