Love can come and go to crazy people, and Bill is a good demonstration that this is true.

He was crazy and sickeningly in love with you, for him, nobody else existed except you. And he believed that this should be the case on both sides.

Everytime you were seen with someone that wasn't him that person magically disappeared without a trace. Where would they go? Directly to Bill's murder list. He was capable of doing anything for you, absolutely anything.