Shamrock Rover
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execute someone, but it's unclear who, and if the execution is wrong, the town loses. And if you don't execute, you have to rely on the random actions of the serial killer, and if that random action is unfavorable, the town loses again.
That's almost always the case when there's only one mafia member. It requires flawless play and a bit of luck.
Added after 31 seconds
But it's better to decide on something, after all.