Is there any reason the SK has not murdered yet?
We'll assume it's not a complete lie that there is a scum named Joe Rouerk disguised as someone else in the group; the person who came up with this is actually telling the truth about their personal goal/role PM.
It's possible the thief and this serial killer aren't the same person, I certainly don't have enough info to judge for myself and only have to trust the judgment of what I've been told. Maybe the "Joe Rouerk" serial killer really is just someone else who was completely inactive and never sends orders.
Or it's possible to be anti-town without having a personal kill ability, and the thief/SK role can't kill or won't until some unknown condition is met as mentioned. The thief is certainly against us, there's no other explanation.
I mean, we're not seeing any nightkills at all in general, but I'm pretty sure there are anti-innocent scum out there, either lone roles or a group. I'm still good with lynching, especially inactives who have no defense of themselves; the difference between like 15 or 17 innocents alive can't matter in any way we know while catching a scum is certainly worth it if we do.