Yeah, I don't think people are clinging on to anything when they bring up a sexual/romantic motive to these murders.
I believe he may have been angry about some criticism or perceived slight re: the lab, but I honestly don't believe Annie would be dead if this guy didn't have some kind of fixation on her. I think it'll come out that the motive was very complex. Maybe Ray felt down about his life, uncertain about his marriage, paranoid about his job, and saw Annie as someone who had everything he wanted--not to mention she was attractive and due to get married to another guy who was probably someone he wanted to be.
Whether he wanted to have her or wanted to BE her, I think he was jealous and obsessed. I also wouldn't be surprised if there was some kind of sexual assault, but as we know that's more about control than sex.
I just think there's a lot more at play here than a lab tech going postal, for lack of a better term. Why would Annie be the only target? Why wouldn't he have come in, guns blazing? Why such a physical crime?
The only other scenario I can think of is that she was threatening to go to his boss. Maybe she had given him warnings. But remember the article citing the email where HE confronted her and she responded in a concilliatory way. Why the about face?