8:31 AM PT -- The parents of Baxter's estranged wife say the rugby player was in a "downward spiral" prior to the murder-suicide ... claiming it all started when Hannah left Baxter after 11 years of marriage.
The parents detail Hannah's relationship with Baxter, saying he was controlling and abusive ... and believe he hacked Hannah's phone so he could track her every move.
The parents question how Baxter was able to get so close to Hannah during the attack ... explaining he wasn't allowed within 65 feet of her following a recent assault, and there's no way she would have allowed him to come close.