i think he was a sociopath, and all his lies were catching up to him. he got backed into a corner in his life where he realized that he wasnt going to be able to keep up the charade, and something had to give. something sparked his panic response and he probably strangled her. he may very well have seen it coming, knew it had to happen eventually, since the baby was almosot due, and partially planned it ahead of time, or rather, prepared for it ahead of time.