The players who hold the red cards are the good guys and the player who holds the black card is the bad guy, or called killer. This game is played between the good guys and the killer. The interesting thing is each player only knows the status of himself or herself: meaning being a good guy or the killer; each player does not know the status of the rest of players. Yet the good guys need to come together to find the killer.
Basic rule of the game: if the good guys find the killer then the good guys win and if the killer kills all the good guys then the killer wins. Depending on how the game goes, a game can typically last between 10 minutes to up to an hour.
Now with all the players knowing the status of their own cards and with all the cards put on the table, the game will start. During the course of the game, each player will need to follow the instructions given by the judge.
Now the game starts:
Round 1: the judge will ask all players to close their eyes so all eyes are closed except the judge. After making sure eyes of all players are closed, the judge will then ask the killer to open his or her eyes. The killer will do. Then the judge will ask the killer to kill one good guy. The killer will very quietly point to the judge one of the eight good guys. Now the judge knows which good guy is dead, the judge will ask the killer to close his or her eyes. Then the judge will ask everyone to open their eyes and after all eyes are open, the judge will announce which good guy is dead. It's worth mentioning that during this whole process, from the judge saying "close your eyes" till "open your eyes", all players including the killer must be as still as possible so as not to let the rest players know his or her status in order to have a fair game.
Now with everyone knowing who is dead in Round 1, the discussion starts. Each player should carefully observe the rest players to determine that one killer. Typically the killer is nervous as he or she is hiding among the good guys and trying the best not to be found. And because he or she is nervous, the killer tends to behave a little different than usual. If a person is not very good at lying or hiding or blending in with the rest, and this person plays the killer for the first time, the good guys might be able to quickly detect there is something not right about the guy and quickly exposes him or her. At the end of the discussion, there will be a vote participated by all the players to vote one player they think it's the most suspicious to be the killer. If the vote is correct, meaning the vote is the killer, then the games finishes and good guys celebrates winning the game against the killer.
But in most cases, it's not easy to be able to point out the killer at Round 1 because it's tough to tell. That means the voted-off guy is a good guy and the vote was wrong. Now there are only seven good guys and one killer left and the game continues on to Round 2.
Before moving on Round 2, let's take a note about the judge. As the judge saw the killer so the judge knows who is the killer and where are the good guys. To have a fair game, the judge needs to be as impartial as possible so as not to favor neither the good guys or the killer. The judge should also be professional in monitoring the game. Restraining from talking, showing emotions, or sympathies so as not to leak information regarding the status of each player. In most cases, the judge should just monitor and observe the game.
Round 2: it will be almost just like Round 1, judge will ask everyone to close their eyes, the killer will kill one good guy, the judge will announce the death, and discussion starts. By Round 2, the good guys should be able to gain more clue as to which one is probably the bad one by piecing info together: who voted to kill the good guy in Round 1, who's saying what, etc. And there will be a vote for this round. If the killer is picked out by the vote, the good guys win. If not, it goes on to Round 3, 4 till at the end.
Ending: if at the end, all the good guys have been killed by the killer, then hats to the killer, he or she is the best liar and the killer wins. On the other hand, if remaining good guy(s) is able to pick out the killer. Then the good guys win.
The above discussed game is the simplest version of the Police and Killer game. In complicated versions, there can be two killers if there are more than twelve players. There are also versions of the game that includes policeman.