I really don't like the whole "slake her lust" theory. No one is so cold to kill someone just to remove a temptation. That is symbolically what she's doing, but not her true motivation. I think her true motivation was to save everyone else, but she chose to kiss him just so she could justify it. "Well, I had to do it that way." It's all about making excuses. Jack makes them all the time in regard to her up until he is kissed and he realizes he can't take his eyes off this woman who's killing him. I don't think Liz has just been lusting after Jack like one of the writers (can't remember which) keeps insisting. I think all of DMC was figuring out who she is and who she is best suited for. Being with Jack in her case isn't just about passion (although there is A LOT of that), but it's also meeting someone similar, your other half, and she keeps hoping he's a good person because she wants to be attracted to a good person.