There will be matches during your playing career when you happen to get stuck with a player who should take an anger management class (or two). This will always happen when they are behind in a match. (The same individual never acts this way when they are winning. Go figure.)
There is one very good reason to be polite to someone acting like that - you already have him beat. The only way you are going to lose is if you break both arms, one leg, and get blinded in one eye.
Regardless, you are still going to be faced with his actions throughout the match. His activities include loud negative comments as you are shooting. He will also attempt very crude sharking efforts such as banging his stick on the floor, moving into your line of sight and attempting distracting movements.
If you think those are fun, you should watch him acting up when he comes to the table. Every time he misses is immediately followed by loud complaining about the table, the stick, the temperature of the room, and any of a dozen excuses for failure.
If the person doesn't have teammates to keep him under some semblance of civilized behavior, you might also have a chance to see him start abusing his cue stick. (This is always good for some entertainment value.)
There are several ways to handle this type of person. If you are playing the match as a member of a team, you can pull his captain off to one side and ask him to caution his teammate. If a friend or friends of his are watching, ask them for some support.
You can also be more direct and ask point-blank, "Do you want to forfeit this match?" If he answers to the negative, say something like, "Please show some sportsmanship. Everybody has bad days. OK?" If you can get a grudging agreement at this point, then finish off the match with as little direct communication as possible. Properly shake his hand at the end and exit stage right.
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