Check out Eric M. Bergerud’s book <em>Touched with Fire: The Land War in the South Pacific</em>. It does a phenomenal job at showing the reader that the environment is as much the soldiers’ enemy as the enemy himself. Malaria, dengue- and yellow fever, fungus, sword grass, snakes, and even crocodiles harassed soldiers in the Pacific as much as opposing armies.
Touched by Fire, by Eric Bergerud is a very good book about the battles in Guinea and The Solomons. Very recommended! https://www.amazon.com/dp/0140246967/ref=cm_sw_r_cp_awdb_imm_t1_j-QTFbYBDRA40
It's true that Bonzai attacks were an and that Japanese soldiers were indoctrnated, but this book is one at least cited as containing accounts of US soldiers becoming fearful to take prisoners and executing them instead: (https://www.amazon.com/Touched-Fire-Land-South-Pacific/dp/0140246967) and then there's footage, such as what is mentioned in this article:
https://www.theguardian.com/world/2001/jun/03/humanities.highereducation
I don't think US soldiers were generally doing this out of spite, but rather because they had been in situations where Japanese soldiers feigned surrender and then attacked.