How does the corpse sitting against the tree affect the tree affect Henry? Is that the reaction you would have? Why do you suppose the author refers to the body repeatedly as " the thing"?

From the text:

The youth gave a shriek as he confronted the thing. He was for moments turned to stone before it. He remained staring into the liquid-looking eyes. The dead man and the living man exchanged a long look. Then the youth cautiously put one hand behind him and brought it against a tree. Leaning upon this he retreated, step by step, with his face still toward the thing. He feared that if he turned his back the body might spring up and stealthily pursue him.

This question calls for your opinion or reaction. I likely would not have screamed, but I would have been shocked and likely turned to stone.... briefly. I would not have been frightened.

In answer to the last part of your question, the body was a thing. It no longer lived.... it's heart didn't beat.... it had no dreams, no feelings, no desires, no regrets. It was simply the shell of a man, who once lived.