Is brought to us by the always helpful Jimmy Akin. In short, you can't want to murder people, and you can't put loyalty to anything above devotion to God and the preservation of human life. With that said, this also includes stepping in and possibly hindering a person's ability to protect himself which is a right the Church affirms.
As a note, when I was a tyke (c. late 1960s), my Dad was driving home from work one night. He worked late and came home in the early morning hours. During that period, there had been a string of crimes committed in the rural areas where we lived. Mom kept a sawed off 12 gauge shotgun next to the back sliding door that was our main entrance-way. We left it alone because we were taught what guns did, and we listened to our parents. I would rather have been shot than disobey my Dad. Only when guests were around did she put the guns away. We were that careful. Plus, she never slept anyway. She would wait by the door until Dad came home (she didn't like it out there in the country at night). The door faced the driveway.
Anyway, that night Dad was driving home down the old, gravel road. Suddenly he noticed a group of young men standing around a car alongside the road. He drove past and saw in the mirror that they all piled into their car, swung around, and began following him. He sped up. They sped up. Before long it was a full blown drag race down back country roads. Fortunately Dad had been a bit of a hell raiser himself in his day, and could handle a speeding car rather well.
Because of the direction he came from, Mom saw him raising the cloud of dust against the farm lights down the road. She knew he was driving fast and assumed something was wrong. She slid open the door as Dad swung into our driveway. Up to the end near the barn he came, slammed it into park, pulled out the keys, and ran to the door yelling at Mom to give him the gun. The car full of punks pulled in behind him and came up the drive and came to a halt.
At that moment, Dad had the gun, swiveled on his heels, fired a shot into the air and proceeded to march straight to the car. Without a moment's hesitation, the car flew into reverse, out the driveway, down the road and was never seen again. It's worth noting that according to Mom and Dad, there were no more reports of the crime wave after that.
So there, we have an example of a gun being used to protect someone, namely us. Sorry if our existence skewers the mockery of those who have used guns to defend themselves. Which is why the Church actually affirms our right to do so. None of this is to say there can't be, or shouldn't be, legitimate, balanced, and serious debate on what we can do to reduce all violence, including gun violence. It is to say that anyone even coming close to saying their approach to gun control is the only option or you hate Jesus is actually the one who mocks God and blasphemes the name of God before the Gentiles.