Habah l’hargecha hashkem l’hargo.
Laskern, the posuk clearly states you should kill him, not wait for someone else to do it.
And you will need a gun to kill a gunman, so it is not bitut torah to get a gun. And having a gun without learning to use it us clearly a lack of hishtadlus.
Hiring someone to protect you is another option, just like hiring a mohel or teacher for your kids, or hiring someone to learn torah on your behalf.