He takes the field in a trance, repeating them to himself.
A major theme of this passage is the idea that remembering brings an event from the past into the present. Wolff clearly uses past tense for events that aren't remembered, but Anders' recalling of that day many years ago brings it into the present, because he experiences it again as he is dying.
The bullet smashed Anders' skull and ploughed through his brain and exited behind his right ear, scattering shards of bone into the cerebral cortex, the corpus callosum, back toward the basal ganglia, and down into the thalamus.
Shift: time slows
This is what he remembered. Heat. A baseball field. Yellow grass, the whirr of insects, himself leaning against a tree as the boys of the neighborhood gather for a pickup game. He looks on as the others argue the relative genius of Mantle and Mays. They have been worrying this subject all summer, and it has become tedious to Anders: an oppresssion, like the heat
The tense shifts here from past to present. The act of remembering in this story converts an event from past tense to present tense, as the character is re-experiencing the event. The earlier paragraphs were in past tense because he didn't remember them, and so they were stuck in the past. As he is dying, he experiences this memory, and so it is in present tense.
murderous temper.
"Murderous temper" would not normally be a particularly sinister phrase, but with the context provided by the title, it seems ominous