but do you think there's something in his past - a "why he did it" - that could possibly make him less deserving of being labeled a monster? He's called a monster, after all, because what he did was monstrous.
What he did was monstrous, but that does not make him a monster. And he isn't. He is a human being like everyone else. All four of those talking-heads are just as capable and just as likely to do exactly what he did if given the opportunity and the chance to do it. To be honest, there is no way to know that they haven't done it.
Is there some mitigating circumstance that would make what he did less monstrous?
No, but if he was horribly abused as child, it would explain why he behaved as he did. It wouldn't mean that his abuse caused him to behave that way, only that it played a major role in why he thinks and behaves the way he does. I just think that labeling him a monster has very little to do with what he did and more to do with people wishing to pretend, as Wendy Murphy claims several times in the clip, that they aren't capable of such things. And the best way to do that is by making those who have committed those acts not human.