This is something that I noticed and I'm not sure if anyone has posted this yet so I'm sorry if I'm stealing someone's idea but I've noticed that the dead who aren't buried seem to re-animate. Yemi, Christian, both people died off island (yemi died on the plane before it crashed) and their bodies eventually went missing and they started walking around. The people who have died and were buried on island never re-animated. I know that many people have appeared to other people on island (Hurley's vision of Dave, Locke's sweat lodge vision of Boone, Michael's vision of Libby) but these were all shown as "visions" while Yemi and Christian were shown to be "real" and not ghosts or visions but they are actually there. It can be inferred that they have been taken over by the island. Maybe this is a reason the truce says that bodies must be buried because it is known that they might just come back to life if they are not properly buried. It might also be why Amy was so stuck on bringing Paul back with them because she was fearful that he might become a "servant of the island" or something. It might be also be why Richard wanted the body as retribution, Paul could possible be brought to life as a "servant to the island" as all other bodies on island have come back as "servants to the island", like Locke also. The losties have buried everyone that died while at their camp: Ethan, Ana Lucia, Libby, all buried and some have even "appeared" to other people as visions but they apparitions of the island. Christian, Locke, Yemi were all really there. Any thoughts? Again, sorry to anyone who might have already had this theory.
--Edit--Sorry for the bad title post, I know that I state the exact opposite in my post, so sorry.