I have wondered the exact same thing. I think he was faced with a real problem because she didn't die immediately and he had concerns she might be able to somehow communicate with the police, or her friends (if they had found her) that SL had been in the home that evening. I also think he planned on slipping away from the house when the deed was done, in the cover of dark, while neighbors were still sleeping, so as to avoid being seen. But when she didn't die immediately, I think he waited until he felt she would not be able to communicate and a new scenario was necessary for the cops because by that time, a little after 6 a.m., neighbors were starting to get up for work and he may have been worried he'd been leaving the house by a neighbor. These are my thoughts, but of course this is pure speculation.