Endymion having written:
<p.63 "What did you want with your brother?" the man of the Vanished People asked him. And he said that he had hoped to bury the corpse and pray for the dead man's spirit. "So I feared..."
<Why did the Neighbor fear that Horn would try to do this??>
Here's my interpretation, but it involves a little bit more than the few sentences you cite.
Because when earlier on page 63 the Neighbor says, "There are two of you," Horn assumes he's talking about both himself and the cadaver in the river, whereas I believe he's actually referring only to Horn, who has some sort of dual aspect (just like the Neighbors). A transformation has happened when Horn was trapped in the island pit, I contend, and the Neighbors appear responsible. Here perhaps the Christian analogy works well: Jesus the man is crucified on the cross, but it's Christ the God who emerges from the tomb. Similarly for Horn, who emerges from the pit as Other: something-plus-Horn, who's able to transport himself across dreamscapes and create swords out of thin air, etc., etc.
This Other then may be "the brother" referred to by the Neighbor, who fears Horn will attempt to sever himself from his own human roots; or it may simply be the cadaver in the river, who, if Horn takes the time to bury and pray over, will delay his sewer-cleaning mission. "It's the past holding on," as the old blind man tells him shortly--whereas Horn, in the hopeful mindset of the Neighbor, needs to move on and fulfill his larger-than-human destiny.
Robert Borski
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.