The rage factor is hard to determine. I agree that if it exists it would definitely point to the parents, but I can't make up my mind if I think the attack was methodical and well targeted or some kind of anger/rage. The problem I have with rage is that the kids were stabbed only enough to kill them. I would expect someone out of control to stab many, many times, like a dozen or more.
As for why she stabbed Devon so deep, as I recall the blade penetrated on an angle so part if it might have been the position both of them were in. Also, it appears he was fighting back, at least briefly and that might have angered her enough to plunge harder with the next stab.
I try not to visualize the attack in too much detail for the same reason you mention. It is difficult to imagine and makes me shudder if I even get close, so no thank you on that one. I will let someone else play the Jordan Cavanaugh game.