In California, a person is justified in using deadly force to protect another person if that other person is justified in using deadly force. (That’s the law in most, if not all, states.)
As a Californian, I understand why people were unable to stop him. In this state, virtually the only people carrying guns are criminals and cops. That means if you see some bad stuff going down, and you step in, you're putting your life on the line with no means of defense except for your fists.