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.
Oh, but I think choice is what she did have - and that's what made her special. She saw that working for WRH gave her what she needed and wanted, and she grabbed that. Lilah didn't care that they spun her moral needle, she didn't go there to do good - in fact, I don't think Lilah actually even believed that there was pure good.
She was fiercely amoral and completely out for herself - completely grey , with no desire to be a white hat. That's what made her refreshing to me. The other characters all had moments of wavering, touching evil but with an unrelenting desire to be good that pulled them back from the brink. Lilah was cynical, and unashamed, and she liked being in charge. That's why her interaction with the fang gang were so great - snark with substance.
I think the scene that showed her relationship with Wesley best was when she played him:
LILAH
Ooh. A drop-by. That's a surprise.
WESLEY
Is it?
LILAH
Well, that it took you so long, yeah.
WESLEY
(walks inside) You played me.
LILAH
You played yourself.
WESLEY
On the phone, you wanted me to hear that so I would tell Angel.
LILAH
Free will. Look it up.
WESLEY
Lilah...
LILAH
I was just doing my job. You're the one who decided to take what you overheard and give it to the good-n-plentys. So before you go all righteous fury, figure out who you're really mad at here.
WESLEY
(sighs) What was the real plan?
LILAH
Let's just say, I could've had Lorne's brain in a jar, but I left it in his head 'cause he's a friend of yours.
WESLEY
What's that supposed to do? Lull me into trusting you again?
LILAH
If I'd thought you'd ever trust me, I would've never played you like that.
WESLEY
It's never simple, is it?
Add that to the scene playing in Wesley's mind as he cuts her into pieces - oh, shivers!
LILAH
I know what it is. The reason you're having such a hard time with this. Why you're taking so long to—you know... (makes a creaking noise as she gestures across her throat with her hand) The awful truth: you couldn't save me. And this is the exclamation point.
WESLEY
Saved you from the Beast, for all the good it did.
LILAH
(smiles) Wesley, you know that's not what I'm talking about. You couldn't save me from me.
WESLEY
Is that what you thought?
LILAH
Me? (laughs) Lover, I'm not even here. I'm just a figment in your devilishly handsome head. So, clearly, it's what you thought. For all your supposed darkness, edge of the razor mystique, (whispers in his ear) there was always a small part of you that thought you could pull me back from the brink of my evil, evil ways. Help me find redemption.
WESLEY
Redemption?
LILAH
(stands behind him with her hands on his shoulders) Angel's influence, I suppose. The whole not giving up on someone, no matter how far he—or she—has fallen. Oh, well. Too late for me. (Wes stands) Let's just get it over with. That body's not gonna dismember itself, you know.
WESLEY
(lifts the battle-axe) I'm sorry, Lilah.
LILAH
Oh, Wes, we don't have that word in our vocabulary. Not people like you and—
Wesley swings the axe. When he pulls it back, it's got blood on its edge. He cries.
Lilah knew Wesley better than he knew himself - and she loved him but neither of them really could change for the other. There really wasn't a 'people like you and I' because Wesley really was a white hat. I like that kind of doomed - the kind that comes from internal tension instead of the romantic UST.