Because of Mikey's choice of the word "assailant".

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The caroler went to sing to the "assailant" after he had already killed Blackmage.

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The Sleepwalker (Blackmage) has no control over who he visits. He also doesn't know he's a Sleepwalker, just like the Loudmouth(LM) doesn't know he's the Loudmouth.

Blackmage visited the Mafia in his sleep, the caroler ALSO visited the Mafia member who killed Blackmage later on that night. I believe Mikey said something like "an hour prior" or something like that in the beginning of the next paragraph.

Unless Mikey doesn't know what Assailant means. Then my theory is completely off.