When will people realize there's always a Civil Arrest or Citizens Arrest? (Whatever it's called.)

If I ever saw someone do that I would detain them until authorities have arrived.
That and if there was ever a gunman shooting up or someone slowly murdering another person I'd try and stop them. There's no real reason to Not try and run away. (Seriously, you can either be a hero and die trying to be a zero and die doing nothing to save other people's lives.)