The definition of murder is: taking the life of a person who is not immediately threatening your own life or the life of another.
Under the definition Paul Hill is only a murderer if you deny the personhood of an unborn child.
If you are working with a different definition of murder than my definition, please spell it out.
Non-Sequitor
You don't seem to be applying your famed Catholic 'right reason'. If your conclusion follows than any man may take on the function of the State in those cases in which he believes the State fails to enforce a moral law. These sorts of actions cannot be randomly applied by individuals but must be through communal action.
Using your definition, Paul Hill would be guilty of the murder of Col. Barrett, who was not immediately threatening the life of anyone.
Yes, Paul Hill knew that Col. Barrett was often armed, but Col. Barrett was not "immediately threatening the life" of anyone, and one would think that Paul Hill could have killed John Britton before Col. Barrett might have shot him back, since Hill had the element of surprise on his side.
However, Hill shot Barrett first (so Barrett wouldn't have a chance of shooting Hill, even after Britton was dead) then shot Britton, then put his gun down so the police wouldn't shoot him.
Hill might have had the courage of his convictions, but he certainly didn't intend to die for them that day. Instead, he murdered Col. Barrett.