IMO it is threads, discussion and the general visibility of threads like these which provide some of the response they see as justification for their actions.
They had a message they want to broadcast and will do it in as visible a way as possible. They use explicit imagery of murder to achieve this objective. And it works! We are talking about it. It is plastered all over the news media. Twitter, Facebook etc are all talking about it to a greater or lesser degree.
If our responses (by our, I refer to us as a general population and the news media, politicians etc) continue to be as vocal and outraged, then we are helping to elongate a self-perpetuating cycle.
I’m not saying that these things aren’t noteworthy or indeed the victims do not deserve to be mentioned, but I do thing that they have no reason to change their approach as it is working to their ends