I don't want to go on 100% gut feeling at the moment; so I'll stick with around 90%.
The killer wolf thought that Mormegil was the rival wolf.
It thought this because Mormegil was playing similarly to itself. (Almost a quieter version of its normal self?)
So was anyone similar to Mormegil?
SPM: Seems normal enough. Lots of big words, attracting rivals. No.
TP: Dominating the village. No.
Diamond: Being overly silly and worrying. No.
Anguirel: Pretty standard, making me laugh in every post. Only scathing in humorous way. Possibly.
Roa: Usual detailed analysis. Probing as ever. Maybe.
Kath: Too quiet. Definitely maybe.
Mithalwen: Not cautious. No.
LMP: Cantankerous, like he says. A trick? Maybe.
Nogrod: Standard. Speaks sense. Got involved. Probably not.
Esty: Standard. Too playful. Worrying me. Not like Morm, though.
Eomer: The most similar to Mormegil out of everyone here.
My conclusion?
Hopefully I lost you ages ago and you didn't see me point at myself.
I'll vote for Kath today, though I began the post thinking I would be voting for Anguirel. My guess (not entirely without a couple of floorboards to stand on) is that Kath, trying to destroy her great rival, smote down Mormegil because his behaviour on Day One was safe, secure, not-too-involved, and not-too-suspicious. She saw her own strategy mirrored, and thought to herself "I shall feast on wolf-blood tonight, and so I shall have the village to myself without a nocturnal danger."
++KATH