I suspect, he has many inner weaknesses and the persona he presents to the world is not who he is at all. I think he is angry because he had to pretend, in many ways, for so many years. I also think that his inner core is so much at variance with the expectations of what a "good family man and a good Christian is", be it Indiana or Salida, that he is scared of anyone seeing it. I think the ultimate reason for the murder, if he indeed is the murderer (so far, formally, the chance is still 53%), is the need to protect himself from being seen by the world without the mask.
We have many cases coming from Colorado. The reasons are different. CW killed for a new love and the perceived prospect of having a new family and new children with her; PF, for the whim of (his own, or someone else's) of raising the child on his own (mixed with heavy paranoia). Both guys behaved like guys, one treated family like old shoes he threw out, the other one, killed and called a woman to clean the mess.
Whoever killed SM (and I believe she is dead) is very different in his thinking. This is why she can not be found - yet. I hope she will be. And I don't think it was just a crime of passion; or if it was, the passion had been condensing for years, and it was not only against her. It was against the world she, in his view, represented.