The key thing to remember with these horrific crimes is that you're not supposed to be able to understand how someone can commit them!
When I think of how much I love my little kids and what I would put myself through to prevent harm coming to them it just sickens me to the core to think of someone doing something like this.
Especially the cases of prolonged abuse. Or some sick fuck like this, who went through the (long, stringent) process of adopting a child only to kill her two weeks later: http://www.telegraph.co.uk/news/2017/11/06/fatherconvicted-murdering-18-month-old-adopted-daughter/
Sickening.