I wasn't there, but it makes sense to me. The two rounds to the head were fired after the gunfight was finished. It is not hard for me to see that how Dallas saw it as the only way "cleaning up" the situation as reaction to what had happened set in.
BTW, if he had not fired those shots, it is likely he would have been acquitted. They were a big mistake. He should have just submitted to the officers and paid his fine, instead of worrying about his livestock.
You seldom have upstanding members of the community involved in cases such as this, which is what ties Gordon Kahl, Claude Dallas, and Randy Weaver all together. They were all well thought of in their communities, which is why the cases became controversial.
I rather doubt this is a self-defense matter. Police routinely draw their weapons especially when it was obvious that the suspect was wearing a pistol while being guilty of a crime.
put them out of their misery,,we do it all the time.