I'm pretty certain your boss didn't tell you to beat up shoplifters over candy bars. You would be in trouble, yes, because you can't use that kind of force, potentially doing real damage, over a $1 item.

Not sure why anyone who is not paid to provide security would risk being fired, arrested and sued, if not also beaten or killed, for trying to protect someone else's petty property.  No, you're not liable for loss due to theft. Yes, your actions outside of company policy will mean you get personally sued and the company doesn't have to pay for your legal fees.

your boss has no right, in most states, to tell you to injure or assault anyone

If you live in a blue state you will go to jail. If you liver in a red state you have every right to protect your property.

Some states allow you to use reasonable force to detain a shoplifter. No state allows you to "whip their butt". Your boss can not instruct you to do something illegal, so you could be fired and/or charged with assault. If you are acting outside of company policy, you could also be sued by the shoplifter. 

Yes, you go to jail for assault and battery.

Legally it's assault because stealing a candy bar is not LEGAL justification to "whoop someone's butt."

In addition to ASSAULT, you can count on being fired FOR CAUSE.

I can also guarantee that if your boss actually told you that physically assaulting shoplifters was acceptable, and you can prove it, they will be out the door faster than you.

Yes, you may be

Your boss might be in trouble for instructing employees to assault shoplifters. You, too, might be in trouble for committing assault. You have the option of choosing not to obey an unreasonable order.