It's never explicitly spelled out, but I think we can assume Abdullah's main motive was greed. He was apparently stealing or planning to steal artifacts from the site, like that jackal amulet, to sell on the black market. Most of the workings of the "curse" were his attempt to get the ethical people out of the way while he stripped the place, or so it seems.

We can also guess that he kept Lily on the site because he knew he could blackmail her into cooperating. Or was it just that she was so weak-willed and unethical that he knew he could manipulate her into doing what he wanted?