Wesley didnt become a jerk because of Lilah. He was who he was regardless of her. He was that way because of what he did and his friends abandoning him, that had nothing to do with Lilah. Wesley was alone and depressed and angry, very much like Buffy was in season 6. He used Lilah because he felt he didnt deserve better like Buffy used Spike.
Lilah loved him, no not in the same way spike loved Buffy, but she loved him in her way and she really wanted to be with him she just couldn't show it because she didn't how to show her feelings anymore. But what she wanted was Wesley just like Spike wanted Buffy. And like Buffy, Wesley eventually loved Lilah though he couldn't admit it to himself.
You may not see it, but to me Wesley and Lilah are very much the same as Spike and Buffy. Of course theyre not exactly the same no two couples are.