I'm pretty sure if you take his king and he doesn't know he is in checkmate, it would spark a fight. Just get someone who knows the game to referee, so the guy would know that he's a loser. Answer The rule of the game is that when a player places another player in a state of checkmate it is announced - so a person should always know that he is in checkmate. By tradition, a king is never taken.
The correct announcement of "Checkmate" ends the game. To actually take the King would be considered rude and insulting in some circles.
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Not normally.
In most standard games, the player who made the illegal move must retract his move and play another one. If the piece he moved can prevent the check, he must move that piece. Otherwise, he may make any legal move he wishes.
If the person who didn't make the legal move has less than 5 minutes remaining on his clock, he may claim an extra 2 minutes should his opponent make an illegal move.
Sometimes in blitz or lightning (buzzer) Chess, taking the king is allowed.