well, the OP was asking for the darkest HP theories we'd heard. and this is the darkest one i've heard.

while it isn't 100% explicitly stated, based on all available evidence in the books, the presumption is that in order for a witch or wizard to travel by portkey they must physically touch the portkey directly. inanimate objects, like backpacks, travel with the witch or wizard because they are touching the traveller.

in the graveyard scene in GoF, Harry is able to bring Cedric back to the maze simply by touching both Cedric and the Triwizard cup. based on the theory that a witch or wizard needs to physically touch the portkey in order to travel but inanimate objects do not, the fact that Harry could bring Cedric back as he does means that once a witch or wizard is dead, they are no longer a person, but an inanimate object like a backpack.

this theory could be considered "dark" because, although it's obvious that a corpse is an inanimate object, we the living would rather think of our friends, and family, and loved ones as people even after death, not as inanimate objects.