‘how does someone get to the point in their head where they justify an action like this’
It’s easy and you can see the starting of it on many threads on here.
Just demonise/dehumanise a whole section of society (lefties, tories, fatbikers, 29ers, whatever) for their beliefs/behaviour, add in a little bit of mental illness/environment/psychopathy/etc and it goes a step further to the point where you’re willing to kill.