I think it's probably Thaddeus that murdered him, when he's interviewing you he mentions killing someone to further his own goals as a test of your lie detecting abilities, what if that was a hint, a laugh in your face, "I'm the one who killed Marek and you don't realize I'm telling you", then when you say he's telling the truth he backpedals to remove any suspicion. And the murder happens on the day he arrives/the day before he "officially arrives" (can't remember if he's there the day Marek is murdered), after all if he's planning something Marek would've been the only one in the village that could've figured it out.