I think this is related to the problem of underlying axioms. When you accept an underlying axiom, you can debate about degree, but you will have excesses following the axiom. The underlying axiom at work here--violence can be a moral means to prevent evil--is largely accepted. So when some view a particular action as abhorrently evil, they may consider violence to prevent that evil as morally justified. The same logic that allows some to justify war (the general principle, specific wars, and particular practices of warfare), torture, and capital punishment, can be used to justify murdering.
Many grapple with this underlying axiom to try apply it responsibly in a complicated world. Some abuse the axiom for their own ends, while others ignore it and use violence to their own ends. Some will apply the maxim in ways that most of us find dangerous and immoral. And some (very few) reject the principle altogether as a matter of ethical principle.