are most people who commit violent crimes sick or just evil?

Answer 1

You forgot desperate. Some are junkies or reached the end of their rope and see no other solution to their problem. of course this still doesn't negate sick and evil as additional factors.
Its been my experience with homicide suspects that they usually either knew the victim and it was some mixture of jealousy, rage or motive like money, property or getting rid of the spouse/girlfriend because someone else was waiting in the wings.
I've also run across a couple...but rare..of the people who have no remose and never batted an eye when they killed their victim. No more remorse than you would have swatting a fly. Those are the people that need to be executed. They're a danger to society alive.

Answer 2

Most people that commit violent crimes are lacking in good decision making skills, have no morals or values. Often lacking respect for people or property. A moderate number of criminals have mental health issues, that have not been diagnosed or treated. A small percentage are, as you say "sick" others are just plain evil.

I have worked in the ohio penal system for 20 years. I have interviewed hundreds of convicted felons. Crimes ranging from small time thefts, to robbery and rape to mass murderers.
In some cases these perpetrators were under the influence of drugs and alcohol. A fairly large number have documented cases of mental illness.

These prepetrators have no regard for anyone but themselves. They derive a sense of power when they are committing these violent crimes. Even after they have been convicted and sentenced and placed in to correctional facilities; These perps continue their pattern of behavior.