How does the corpse seated against 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"?

Henry is shocked into reality when he encounters the dead man seated against the tree. He is shaken, as the man's body brings the horrors and loneliness of war to the forefront... the corpse sitting in from of him serves as a symbol of everything he has been trying to avoid.

The second part of your question calls for your opinion.

In context, the author refers to the body repeatedly as "the thing" because for Henry, its humanity no longer exists.... Henry remains unable to confront the realities of war and its toll on human life.