"I would not feel safe if I was sitting next to someone on the train or bus and they were carrying a weapon," said O'Brien. "I don't trust a stranger to protect me, let alone trusting them to carry a gun," he said.
See, that stranger is not there to protect you. If you were to ask them, they'd probably tell you they could care less for you, beyond and initial assessment of whether or not you're a threat. Oh sure, if they are a peaceably armed citizen, they care enough about themselves and don't wish for an occurrence of violence to occur to anyone. But you? No, it is not their job. The gun they carry on their person is to protect them.