“Evil” is easy to say, but it is hard to do. (It may be impossible.)
Humans act out of emotions. Many killers kill out of a sense of pride, not evil. They feel wronged; they want to avenge that wrong. Ever hear President Trump boast, “if you hit me, I will hit back ten times harder.” That is the persons a mass killer projects. It is his delusion that, not only must he strike back at the world, but he must inflict a hundredfold grief upon any world that would treat him cruelly.
When President Trump heard about the church shooting in Sutherland Springs, Texas (besides calling the man “evil” and “deranged”, and assuring the world that America does not have a gun problem), he brushed it off as “a mental health problem.” In a way, he is right, except that he sees mental health only in the extreme. (All sins are a mental health problem.)
People suffer mental illnesses on a scale similar to how they suffer physical illnesses from colds to cancer. We all get a touch of madness from time to time. It is called anger and lust and pride and greed. We will do what it takes to satisfy that madness. (Having guns at the ready does not help.) Most of us, once we are done, return to our good and former selves, similar to how we overcome the sniffles. We are not evil. We are simply weak – slaves to emotions we think must be satisfied.
We are quick to label people “evil” when they commit acts we abhor. By our standards, the senator who votes to pull the rug of healthcare from beneath the poor is evil; presidents who levy economic sanctions against poor nations in a deliberate attempt to starve that nation’s people are evil; the child who pulls the wings off of houseflies is evil.
I lack the theological background to put into words the true concept of evil as mankind imagines it. In any case, true knowledge of evil is as unattainable as true knowledge of the divine.
These people who commit mass killings are not evil. They are simply selfish and terribly mean. We make statements attributing evil to them, we are merely satisfying our need to assert control, and display a moral authority we do not have.