Oh, so I'm not allowed to have an AR-15 because someone else used one in a mass shooting?
Stupidest arguement ever.
Guess we gotta ban motorized vehicles now because more people die from them than guns, and that's not counting when they have been used to drive into groups of pedestrians or used as bombs.
Guess we gotta ban planes now since they were used in an act of terror.
Obesity kills a ton of people- ban it!
Smoking kills a ton of people- ban it!
And if the bad guy is using an AR-15, whether he obtained it legally or otherwise, why can't I use one to stop him/her from killing me or anyone I love? That's akin to taking the horns from a gazelle so it can't defend itself from a predator.