IMHO, Amber was the catalyst. (Not her fault if she didn't know he was married.) But people can go along in a marriage for years/decades loving (or not) the person, but having "fallen out of love" (that infatuation feeling) and never replacing it with something stronger. I think he realized that Laci and Conner were 'in his way' when he became infatuated with Amber. He had a cushy life with Laci, he didn't have any reason to kill her until he found a replacement. JMO