I believe the hothead scenario. He simply got uncontrollably pissed over something, and decided to show her who was boss.
Of course he regrets that and concocts the cover story of the burglars. No live witnesses except himself, his word against--nobody else's word.
To me it is first degree murder, but he'll probably get less. Too bad he is trying to use his birth defect, to gain pity in court.