It's difficult to think of a reason they haven't charged him with murder of the baby though, if he knew Kelly and was aware of the pregnancy. I think the intent required for murdering her would lead to the same intent to kill the baby too, so they've charged him with manslaughter because they can't prove he knew.
It does look as if he knew exactly where he was heading though. It's not as if the open window or door was accessible from the street. Maybe he saw the opportunity for unobserved entry at some other time if he was working nearby, but didn't know the occupants and sleeping arrangements.