I do understand wanting to vilify this guy. Personally, he is not the type of person that I would ever associate with, nor do I defend his actions. I do believe that he is a violent man and has difficulty controlling his anger. I thought it was very telling, during the pc episode, that he admitted to smashing the camera to keep himself from "doing something" to MP.
However, I do think it is important to keep a levelhead and remember that, although his past (and more recent) behavior is most definitely a good indicator of his personality, it is not going to necessarily tell us what happened on the day Michelle disappeared. Personally, I think that he had nothing to do with his second wife's death. Her death is suspicious, but to me, the fact that he was in military prison is enough to exclude him. Do I believe he was abusive to MP? Absolutely. There is evidence of this, between her older son's statements and protective order. But, I think making him out to be an evil mastermind is not necessary.
To me, it is very simple. MP went over there to drop off her twins. An already volatile relationship, that seemed to always be on the verge of exploding was exacerbated by the airing of the pc. Perhaps the subject of the hummer came up MP's mom did say he had wanted it back). I think he hit her once with enough power to knock her out or maybe he pushed her and she hit her head. Either way, I don't think it was planned or that it was a lengthly fight. I think it was a moment where he lost control. I don't think he planned it or had any motive, I think he just snapped.