@dcai0830,
@BuffyBot22, Right. Their relationship was complex, love is a multifaceted thing and then the question of vampire love but Darla's undying love/devotion is similar to Spike, his devotion to Drusilia and then Buffy, which gives credence to what Drusilia claimed. Darla definitely hates Angel and despite that, still loves him.
Angel is a Spike mirror in "Darla" "You disgust me" is "You're beneath me", cut to the present day and he still can't let Darla go even though he knows he should, he is a "Fool For Love" and the devotional love Darla has as a "returned vampire/then human/then vampire again" makes her a kindred spirit to Spike in a way.
Angel hates what he is and Darla is what made him that way. Correct, he could never love Darla when he was a demon, at least not what is called pure unselfish love but he is not solely a demon anymore and as we saw he cares deeply for her and couldn't let her go, even when it was the right and sane thing to do, he had a passionate irrational need to save her and it tore him up inside, I'd call that love.
I'd say two people who wanted the same thing, to love each other and couldn't have it is tragic. It's indisputable Buffy and Angel have each others love and have each other as old lovers and old friends who are still around. I am reminded of "People enter your life for a reason, a season or a lifetime" Bangel could be seen as all three. Every romantic relationship one ever has will come to an end, until the day one doesn't, no vampire is required but because Buffy and Angel's day came, does not make their relationship a failure. Whether or not Buffy marries or falls in love again or never does, doesn't change the fact that for the both of them, it will remain, one of their most successful love relationships in its own, everlasting way.
As Harmony told Wesley "The girl you loved, loved you back" I feel as if, does Darla ever get that? I don't think Angel and Darla's hearts ever truly mended and now can't, not ranking the ships but to me it is more tragic. In a alternative version of events, if Angel could in the present day, love and be loved by Darla in the healthy moral sense, maybe he could reconcile himself and hate himself a little bit less