A philosopher once said
In a land where there is no law, life is shortish, brutish, and nasty.
The human mind's ability to do freely what it wants will only lead to a state of chaos because everyone will believe they are right, including the man who kills three children.
Let's put it like this: the parent does have the free will to kill him too, but there is a rule guiding that, but if there is no rule and the killer has family and friends, same with the parent, then its going to be a revenge battle that might not end anytime soon until the two parties are wiped out.
My dear, good enough we have rules to guide our conducts as humans or else things will so fall apart