I never thought that Lilah was as much evil as just a lady who made a commitment, possibly in either haste or ignorance of the entire situation, or both, and, once committed, had no real choice in the matter.
If Stephanie Romanov wasn't such a talented actress, Lilah might not have had the depth of character she displayed. So many times I've seen her eyes reflect pain/vulnerability/saddness and it always touched me. I'm absolutely sure Lilah isn't completely evil. In fact, I believe she's one of the most human characters on Angel.
Funny but Parker was an other character I was thinking of probably being pure evil at heart, yet he did not do what Warren did, (rape, murder, blaming murder on someone else etc.) And we did not see much of him, soI could not say he is pure evil.I don't know describing Warren as pure evil is a bit like describing Parker as pure evil. But it does bring up an interesting question of whether a human character doing such horrible things makes them more evil than a demon, vampire etc..