Thats basically what I was getting at in my previous post. I wasnt trying to suggest there was molestation or anything going on with Emma's family but if Sam believed something like this or the incestuous or possessed etc, as you suggest, it very well could be used as a credible defense and would also allow him to get away with the Pastor's killing because no matter how long it was after he had to 'finish the job.'
I actually wouldnt be surprised if Sam were to eventually make some sort of claim such as these or similar.
Emma and Mel being in to the horrcore thing pretty hardcore could have jokingly or in a mean spirited way, suggested to Sam, that they were going to kill him or have him killed or brutually murdered and he took it seriously enough to wipe them all out and defend himself. If it could be proven he actually believed this, which wouldnt be easy in and of itself, this could work as a defense of some sort or at least lighten the punishment.
Perhaps it was even suggested that the Pastor was on his way over to drive a crucifix through Sam's heart and rid him of his satanic demons and he killed the girls and then bunkered down to wait for his intended target.
I have also wondered recently if Sam could have been involuntarily and unknowingly drugged as a joke by Emma and Mel, maybe some bad acid and a resulting bad trip led to this?