ATPG is clearly not mafia aligned. As a result, his night kills have a chance of killing scum and are essentially as good as vigilante kills; they basically count as a free lynching every day, which is good for the town. At the same time, he clearly cannot be trusted as pro-town and we must assume that his victory condition is town-hostile. As a result, while it may be in the town's short-term interests to keep him alive, he absolutely must be lynched eventually. As I see it, there are two factors to consider when determining whether it's time to lynch him.
1) Is ATPG killing actual suspects? If ATPG's kills remove people who were legitimate candidates for lynch votes, his kills are assisting the town. If ATPG's kills remove people who were not going to be lynch candidates, then he is not assisting the town and warrants lynching.
2) Are there non-ATPG-aligned scum still alive? As noted, ATPG is clearly not mafia-aligned. Therefore if you think there are still mafioso alive, ATPG retains value as a possible way to eliminate them. However, if you think all mafioso are dead, ATPG's value to the town ends.
As a result, you should not vote to lynch ATPG if you think that he is killing suspects and there are still mafioso alive. If, on the other hand, you believe that EITHER ATPG is not killing actual suspects OR all mafioso are dead, you should be placing your votes on him.

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