Even if Amy is the last alliance member, she still needs to work with the town against the Reavers, until they are all gone. With a recruit, they must be eliminated to the last so stop being a threat.
There are seven of us today. Let's assume two reavers, and one alliance killer.
Scenario 1: If we lynch the alliance killer today, then tomorrow, we are at six players, three of whom are reavers, then we lose.
Scenario 2: If we lynch one of the two reavers today, and the alliance player hits town tonight, then we are at two town, one scum, two cult, and in it's lylo for everyone.
Scenario 3: If we lynch one of the two reavers today, and the alliance player hits the other reaver tonight, then tomorrow we will have 4 town, 1 scum, 1 cult.
Scenario 1: There is no tomorrow, we already lost.
Scenario 2: The alliance killer needs to kill cult again to even stand a chance. And thus acts like a vig. If we hit both cult, then on the next day, there will be a three way tie, and town will lose.
Scenario 3: If we lynch the cultist, then and only then will the alliance member be able to start killing town. And the next day will be three town, 1 scum (but we won't know whether we are in this scenario, or whether there is still cult left, and things get really interesting.
Now, all of that assumes that we didn't get all of the alliance for some reason, rather than us having a Vigilante. If our intrepid killer is a vigilante then we are in much better shape.
Again, it is for this reason that we must work together to root out the cult, and only the cult. Any other plan leads to cult win.