If someone's too dangerous to own a gun they should be institutionalized until they're no longer a danger. Just taking guns away from them won't prevent them from being a problem.
Anything that would currently mean a person loses their right to gun ownership. A felony, red flag, whatever. I'm not sure I agree with all of them but the logic of the situation dictates that if a person is so dangerous that they will kill people then that needs to be corrected. Just taking a gun away won't prevent them from doing harm if they want to.
It's hard to argue that guns don't make the proverbial bad guy more efficient at killing. If guns weren't the most effective tool for killing someone, cops would carry cheaper alternatives like billy clubs, and wars would still be fought with swords and bows.
Of course, they do carry billy clubs and blunt instruments are quite capable of killing people too. Sure mass shootings would be harder (assuming we could do one single thing about the six hundred million guns out there already, which, good luck) but single brutal murders w/o guns are also a problem and typically target women, lgbt, and disabled people.