He was not a monster, not a demon or anything else. He was a human being and it is important to emphasise this fact. He was a son, a brother. At some point something happened that sent him down this path of violence and hate.

To prevent others from going down the same path, we need to be aware of possible issues and help those in danger of doing similar things. Demonising them will only discourage them from seeking the help and support.

The same goes for all these people, from school shooters to former Nazi soldiers involved in the Holocaust. They are human beings like you and me, and it is imperative to try and understand what lead them to do these things. Demonisation leads to defamiliarisation and a "they weren't human beings; this can't possibly happen to me/anyone I know" mentality, while in fact it very much can given the right circumstances.