For one thing, even if I did have reason to doubt his claim, I'm leery of simply assuming that we have a serial killer. I'd like to see what happens tonight, though that would require any vigilante we have not to night kill tonight. On the other hand, knowing this the serial killer might also decide not to kill anybody in order to hide its presence, so eh.

But more importantly, fake roleclaims recently have tended to be less basic stuff out of fear that the role already exists in the game and that they would therefore be revealing themselves. This, and the fact that nobody else has claimed Watcher incites me to trust the claim, for now.