In general a murderer is not a crazy person.
When encountering the scandalous, the barbaric, the offensive and preposterous, people often react by naming such acts ‘crazy’ or ‘insane’. They don’t attempt to diagnose the perpetrator. It’s just a manner of speaking, a way of condemning irrational or uncivilized behavior. Or so it should. Popular culture seems to support this view on killers and to promote the idea that murderers are abnormal human beings, whose abnormality most likely stems in their DNA.
There is also the attitude of politicians like Trump, who would rather dismiss the issue of easy access to weapons that can enable people to commit mass shootings by putting the blame on the mental health of the slaughterer in an attempt to circumlocute the real issue.
Human beings’ evolution and history shows that people are capable of murder and they can do it in order to ensure the survival or prosperity of their group, family or themselves. Our murderous susceptibility does ground in our DNA, but it’s something that we all share, not an anomaly. What prevents the number of murders from skyrocketing is social norms and government restrictions.
It is true that certain human beings are more likely to become murderers than others, but that’s not because they’re less sane or human. Human beings show a wide range of types, temperaments, characters, and traits. Some of us may be more aggressive or less empathetic, but that doesn’t mean these people crazy. A sociopath may feel nothing when killing men, women or children, or he may even enjoy himself while doing it, but he can still tell right from wrong.
Most murderers are not crazy people. They have roughly the same DNA as ours, and can tell right from wrong.
(Here’s an expert’s typology of violent offenders.)