The Pacific conflict was an incredibly racial and brutal conflict. Both sides rarely took prisoners. That HBO series is based off of Eugene Sledge's memoirs, "With The Old Breed," and there are multiple descriptions in it about looting the dead of gold teeth, taking the severed hands of enemy dead as trophies, and people callously shooting unarmed civilians and soldiers. This sort of thing didn't happen with 100% regularity, and there were exceptions, but it was common.

Accounts like these aren't limited to Sledge- most with American survivors who were in infantry combat with the Japanese feature at least a few cases of shooting enemy soldiers before they could surrender. War does things to people, and the fact that the fighting was so intense, personal, and racial compounded the issue.