Exactly, you are correct, this guy not only shot his mother in the head, he then went to the elementary school and shot the principal, the teachers, and the children/students, finally turning the gun on himself. This was a deliberate murderous rampage in which the perpetrator was the last to be snuffed out. Demonic possession can work that way, because the Devil wants all of humanity dead, ultimately.

I know many do not believe in such a being. However, lack of belief in something does not mean it lacks reality. If, for example, I refuse to believe cancer is real, does that mean it is not, in fact, real? If this rampage was not, ultimately, caused by a medical condition, drugs, MK Ultra mind programming, or simple anger, then what caused it? There must be a cause. To simply call the perpetrator a 'nutter' is a dodge. Many refuse to believe that evil exists. It most certainly terrifies many to contemplate the cause and source of evil.