Wolfstet
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Sooner or later, Grey will still target the Black faction. No one said he would eliminate only one side. Therefore, he is only useful to the Black faction as long as he cooperates with them, balancing the positions of the factions. And if he decides to eliminate White tonight (from the obviously White faction, or randomly based on a clue), it will actually be a plus for White. Because only Black/Grey will target them, and by solving the clue, another murderer will be eliminated. Which is detrimental to Black.