Someone should determine how many of the murderers come from two parent stable homes?
I am pretty sure that Kleebold and Harris had both parents.
Of course, with a murder team there is a different mental dynamic than with an individual killer.
With a team the disturbed thinking feeds off of each other. Their fantasies are reinforced and amplified by each other.
When the planning is completed and the decision to act is made they prevent each other from backing down.