I have no reason to believe there is a killing neutral in this game. So I will work under the assumption that the only neutral party here is Watson if they are not scum.
I've played many games and I've found that scum try to discredit the legacy of a town leader by crediting it to a neutral or an uncliamed vig. In this case, i don't see why it is unlikely that Poyser was killed by scum, they are responsible for Melkor and was leading town on day 1, certainly that is a threat most scum parties would want to get rid of in most scenarios.
If you believe otherwise, then you gotta ask, why would scum prefer to kill RDK or Arnej instead of Poyser here? Why them instead of loud voices such as Ultra, Flower, Ekko and Professor? The more you read into those kills, the less sense it makes for Poyser's kill not to come from scum.