Why would Gene visit the marble stairs and the tree if they are so fearful? Why are they fearful?

The narrator is projecting. He is describing his surroundings in terms of his own feelings. He describes the November day as "Self-pitying". The attentive reader will soon see that he is self-pitying. These locations are fearful because his is afraid to approach them after what happened there; for what he is responsible.

Also, one is an entirely natural setting and one is entirely philosophical. The "fall" is both natural and philosophical as well. The body is crushed as well as the mind.