Hemingway's mental physical conditions had deteriorated at the time of his death.
Too bad they didn't have anti-depressants back then.
IOW is was nuts when he shot himself.
Happens quite often with people of artistic genius.
For instance Van Gogh shot himself in the stomach and it took him several days to die. Gauguin a peer of Van Gogh also suffered from mental illness and had attempted suicide.
"For when the One Great Scorer comes To write against your name, He marks-not that you won or lost- But how you played the game." -Grantland Rice