After a 21-year-old gunman massacred 22 people in an El Paso Walmart last week, President Trump declared that mass killers are “mentally ill monsters.”

It was a convenient — and misleading — explanation that diverted public attention from a darker possibility behind such unimaginable horror: The killer might have been rational, just filled with hate.

It’s reasonable to think that anyone who guns down 22 human beings in cold blood must be deranged or de facto have a mental illness. But the truth about mass killers and the link to mental health is more complicated than that.

One of the largest studies of mass killers, conducted by Dr. Michael Stone and involving 350 people, found that only 20 percent had a psychotic illness; the other 80 percent had no diagnosable mental illness — just the everyday stress, anger, jealousy and unhappiness the rest of us have.