as soon as he laid his eyes on Bella.
I think Edward fell in love with Bella when he made the choice to not kill her. He may not have realized it then, but he was. Because his plan would've worked out perfectly. He just had to make the action, but he didn't. My only theory to that one is that he cared for her at least a little bit. He had never interacted with humans before she came, but he made the choice to go back to Washington when he was in Alaska knowing that he would have to interact with her--which he did.