Personally, I lean more towards it being the Protagonist.

the Chicken Lady's brother and the father of the two girls ), the Trader might have just been another unfortunate victim. To summarise = the protagonist has a history of hallucinations by this point in the game, with violent outcomes (e.g. The Trader told the Protagonist there is no way out of the forest (he was telling the truth, but the Protagonist, under the control of the forest, being drawn ever closer to its heart, didn't know this - think of the earlier versions of the game, where the man in the tunnel was a soldier who had been one of two soldiers trying to dig their way out after the tunnel collapsed, but one of them went insane and started heading back the way they had came, back towards the heart of the forest, convinced his sane companion was lying to him). The writing 'Liar' would also make a lot more sense this way. Recall the Church Dream, and what happens to the father of the two girls if you manage to kill the black chomper? This is what could have also happened to the Trader. The Protagonist = this is easier to summarise than above. To summarise: evidence is circumstantial, but suggested by timing (plus stolen sack) and possibly the writing. Perhaps the outsiders had once promised the village they would save them and get them out of there? This is all wild speculation of course.

But then again the Talking Tree is also a replica, so why would the Three take offence to this? Could the Three recognise the Trader's military garb as that of an outsider? Perhaps they blamed him for the downfall of the village to begin with the burning of the houses and animals, and then the road blocks, all the while the village grounds had become flooded and they were literally starving to death but were not allowed to leave for fear of contagion. Another possibility is what the Trader embodies himself, a replica of the Protagonist, born of the forest = in a way, a lie, not the true original man but an imperfect replica. Why would the Three take such a disliking to the Trader though? What could he have lied to them about? Perhaps he said something about the Tree not truly being the village, and the Three took offence (but this wouldn't make any sense, seeing as the Cripple says the same, and they've clearly left him alone). It's clear they love the village, having returned nightly after the most of it had died of to visit the Talking Tree. Perhaps in a way it would then make sense for them to side with the villagers that threw them out and angrily blame the people who barricaded themselves in the basement. But that said, unless it was the Three themselves that wrote thieves on these floors, there isn't really a connection (but then again, they could have done = the Three themselves were not actually involved in the raiding of the basement, they were too sickly at the time, it was just their father and a bunch of other villagers, and it's because of this they were thrown out of the village. And it is in their own home (and other villagers homes that barricaded themselves in the basement) that the other villagers have scrawled 'Thieves' in very similar writing to that which is used on the Trader's helmet. The Three = evidence is more circumstantial than anything else, as they appear the same morning the dead Trader does and appear to have stolen his sack (*Note both sacks are said to smell overwhelmingly of mushrooms when opened). It's most likely either the Three or the Protagonist.

Darkwood trader death