What Quincunx said is true, although in many cases the victim and the perp both have a mental health problem. Add in some alcohol, drugs and living on the street and it's hard to tell who did what and why.
There are lots of damaged people in this world who grew up hard, and the majority of them never cause a problem.
P.S. - Decades ago I had a client drop out of sight for a couple of months and the phone was cut off. She eventually called me and said her husband broke her leg in a couple of places with a saw horse.
I said, "He beat you with the 2x4 leg off a saw horse?"
"No, he beat me with the whole thing."
Two alcoholics with mental health problems.