There has always been an element of humanity that willingly-- or unwillingly-- served the Adversary. This element has always had the same basic... I'm going to call it "religious beliefs", even though often the element would deny their being religious at all. These beliefs pretty much align around hating the virtuous, the innocent, the beautiful, and the true, and often involve marring or harming those things, including voluntary personal disfigurement, the harm or slaughter of children, and the destruction of personal or cultural history.

So, the shooters aren't demons, and it's possible that they didn't even suffer from any for of demonic oppression. But they came into contact with the priests of the Death Cult, and they know what their master knows... their time is short, and they must consume while they can.