A theft while at work, on the clock, is handled differently than an assault.
You're right, if an associate is caught stealing by another associate, the chain of action normally taken, starts with reporting it to the manager.
But we're not talking about theft here, we're talking about assault. Also, you stated that you weren't working at the time, you were done with your shift.
You really don't get the difference, do you?
Let's say you and I work at Walmart together. We're scheduled to work a shift together. During our shift I go to the bathroom, you follow me in, and you beat me up. Do you think that I need the managers permission, or okay, to call the police on you?