You are entitled to disagree, now come here so I can beat you with a metal pipe for doing so

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His bad judgement was that he not only put himself at risk, but also his co-workers. Keeping what could possibly be a fake $50.00 bill was not worth the possibility that the girls involved may not have been alone, they could have had somone else with them that could have been carrying a gun.
I have dealt with situations such as this and have had people pull things from a knife to a dirty needle, in one case it was a fake $20.00 bill, the person asked for it back and when my co-worker refused the person pulled out a dirty needle and threatened to stick him with it if the fake bill was not returned. Needless to say I told him to return the fake bill and let the person leave the premise.
People have been shot for much less and it is not worth the grief/trauma that can ensue. He went beyond self defense and is now going to have to deal with a 2nd strike and if found guilty faces the possibility of a longer sentence due to his previous conviction, to me that is bad judgement.
The girls that attacked him, well they will hopefully learn from this(I highly doubt it) but it may give them pause the next time they think they can intimidate/attack a complete stranger. They could have dealt with it in a rational manner and if the money was authentic, they could have called the cpos to get it back, if it was fake they could have left with out confrontation.