I’ve never heard of either being used as justification; not much was known about the Bataan death march at the time and most Americans had never heard of Nanking. It was the IJAs determination to fight to the death and the first surviving encounters with the IJA that set the pace; the Goettge Patrol where only 4 survived from a 25 man patrol when they mistakenly believed the Japanese were trying to surrender, and the first real fight with the Ichiki Detachment at the Battle of the Tenaru which left 777 Japanese dead where wounded Japanese shot at or threw grenades at Marines attempting to provide them medical treatment in the aftermath of the battle. Feeling that these were acts of perfidy by the Japanese, Americans stopped trying to provide aid to the Japanese wounded and simply shot them instead.
Yeah, that there was widespread starvation and resultant cannibalism in the Pacific is pretty well known. The IJA was notorious for being horrible at logistics and repeatedly sending its troops on offensive operations where it couldn’t even feed them. Most casualties from the invasion of India at Imphal-Kohima in 1944 were from starvation and malnutrition. The ideograms used as shorthand for Guadalcanal, Ga-To, had a darkly ironic double meaning; they could also be taken to mean “Starvation Island”. Although the Japanese were able to evacuate most of their remaining troops from Guadalcanal under the noses of the US Navy, they were so malnourished as to be essentially useless.