that artist like to leave her story up to interpretation (check out "the groom" for a good exemple).

add to that the narrator being unreliable, like when he describes commons forest things as weird and eerie (ie: a tree with leaves looking like women's hand while it's just a common oak) or when he said "we both laughed" but you can clearly see that only the brother is laughing.

it's normal to not understand everything immediately, it's one of the comics goal

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the most popular and common interpretation is that the beast wasn't the wolf they killed but a shape shifting creature (most likely responsible for the strange hole in the forest) that used the fratricide as an opportunity to infiltrate the village by disguising itself as the brother, the narrator come back to the hole he dumped his brother in and realized that he didn't actually killed him, so he is faced with a choice: finish the job, bring the brother's body and expose the doppelganger, being the village's hero a second time, or do what's right, save his brother, expose the doppleganger this way but have to face the consequence of his actions.

another interpretation that have less ground to stand on but that I personally like, is that the brother coming back from the forest isn't real, but it is a manifestation of the narrators (already shown to be unreliable) guilt. See in the beginning of the story the narrator define his "brother" by all the thing he have, and when he came back to the forest after having killed the beast (and his actual brother) all of those things are given to him. the "brother" coming back from the forest is the narrator himself, as when he came back he basically took his brother place and now his guilt make him dissociate something fierce. the line "why won't he turn and look at me" is actually the narrator being unable to look himself in the mirror. As I said this interpretation is less likely (although it does remove any supernatural elements) but I really like it, as potentially farfetched as it is