First let me say the boyfriend is in no way innocent, but to me, the MAIN evil here is the female. She was Charles Manson to his Tex Watson. Neither is innocent, but I wonder if some of his reflecting now is indeed "Oh my god, what have I done?" Doesn't excuse him for one second, but I have to wonder if now that he's out from under her thumb, over time he may have remorse? Oh and I do believe you have to "have it in you anyway" to kill even under direction, but I still think the scarier, bigger evil here was the female.
She wasn't one of those who had fantasies that someone else just ran with, she went there to kill with her own hand as well. People who gave birth to her and raised her, a brother to whom she was by reports VERY close to until he married. And they killed the second 4 simply to eliminate witnesses but then wound up coming back and confessing anyway which makes the second series of killings even more useless/senseless.
And why couldn't they have waited until after the visit? Seems they knew the brother's family was coming, so I have to wonder if she intended to kill him as well. If she was only out to kill Mom and Dad, they could have come in the late night or Christmas Day. He's certainly guilty, but she is the one I see as evil incarnate.