Theory time: The tiger could be picking people who're about to die anyway, as I've seen i...

Theory time:
The tiger could be picking people who're about to die anyway, as I've seen in other Korean and Japanese stories that when people start losing weigh and sleep and suddenly being able to see ghosts and the real supernatural creatures even under their disguise, it means they're about to die... (and sometimes it's the work of a ghost or ghosts having attached themselves to their living body).
Second theory time:}
Since the changgwi is the ghost of someone devoured who helps the tiger find his next victim, maybe the mc was picked as next victim by this missing hyung dude's ghost, who was the tiger's changgwi first, and the bitemark is the sign that the ghost left on him as a sign for the tiger.