We're aware the Man in Black could take the form of the deceased. The Man in Black killed Seth in order to take and use his form to perhaps mingle with the survivors. Under the pretense of being Seth, this would have no doubt been a good disguise to operate under and manipulate with towards his goal of leaving the Island. However this did not come to fruition as Jack, Kate and Charlie witnessed Seth's death, so this plan had now become redundant. This may also serve to explain why the Man in Black chased the trio in his monster form afterwards, to eliminate the witnesses so he could carry out the plan. Ultimately he was unsuccessful and with his plan ruined, the Man in Black set into motion further plans we see later in the show.
The monster killed him mistakenly, thinking he was Frank Lapidus.
Based on what happened to Mr. Eko, The Monster seems to be able to identify its victims, even more: it can project images from a person's past.
Maybe his body was never found and appeared as a manifestation/ghost.
It wanted to "flex its muscles" so to speak.
This sounds pretty plausible to me. Just in the same manner as this scene was established by the show's writers to, partially, establish the threat of the monster, this was the MiB demonstrating to the survivors just how dangerous he could be, so that they would be immediately terrified of him if they saw him.--Master Tej (talk) 02:51, December 9, 2013 (UTC)