I don't think he looks cold. I think he looks stoic, maybe resigned, or even relieved.
I think SF and PF had their mutually beneficial partnership established and they were comfortable - she helps him, he helps her. They have lived there quietly for 20 years since he was about 12. Enter KB, and then baby K. PF went quickly from 1 female to 3 females, each with different needs from him. I think he may have gotten to the 'my life isn't worth living like this' point, and the escape route became murder KB. In his mind, he may have seen KB as ruining his life, his previous tranquility and autonomy. He knew the consequences of getting caught would be prison, with men, and may have simply thought that it was better than his situation as it was. He is effectively now relieved of responsibilty for the needs of KB, SF and baby K, and his loss of his animals, rodeo, ranching lifestyle is costly to him, but prison is perhaps less confining, less stressful, and some sort of freedom.
I don't see PF as some sort of all-around bad guy. He has no criminal record, not even a DUI. He has no cases or sanctions from the farrier, rodeo, or cattle associations, or from animal control, or from the State.
I think he came to just hate KB and wanted to eliminate just her. I don't think he is like CW, a family annihilator. I never thought through this that baby K or SF or anyone else was in danger.
I am not defending his choice of action, and not victim-bashing. Like others, I am trying to understand how a seemingly average, quiet, non-criminal guy ends up committing this sort of crime.