Saying, “those people aren’t human; they’re monsters,” both avoids placing blame for how they got that way, and abdicates responsibility for making changes in the future to prevent more people from being like that.
Assuming that “rapists are monsters” means not blaming parents, schools, and the media for teaching boys that “a man’s got needs and it’s a woman’s job to provide sex!” It means pretending that there’s no way we could discourage rape in the future, no way to teach young men not to become rapists.
It also dodges the question of whether some of them could be redeemed, whether they could learn to understand what they did wrong and not do it in the future. But even if you don’t care about criminals’ redemption possibilities, you should care about creating a society that produces fewer criminals.
And we can’t do that, if people claim that criminality is a matter of “evil souls” instead of a combination of societal influences and personal choices, and how people learn to be criminals (or “monsters,” who may be doing horrible things that aren’t technically crimes) based on the consequences provided - or not provided - for their early actions.