Why do the soldiers use the phrases such as "crispy critter" and "roasted peanut" to describe the dead?

These were "funny" often cruel monikers the soldiers gave the enemy,

"If it isn’t human, it doesn’t matter much if it’s dead. And so a VC nurse fried by napalm, was a crispy critter. A Vietnamese baby, which lay nearby, was a roasted peanut. Just a crunchie munchie.”