It had to do with how he was dealing with the war and how the group viewed the Yeerks.
#6: He was still young, not very experienced, and suffered no ill effects from the war yet. He was eliminating the enemy. Nothing more. In fact, if I remember right, he was pretty gleeful about "boiling some slugs."
#21: The peace movement is underway. Jake knows that there are some Yeerks out there that truly want a way out. He's also deep in enemy territory, didn't want anyone to know that he had been there, and David was with him at the time. I'm guessing he didn't want to leave evidence that they had been there, stacked on top of not wanting to look like a barbarian in front of the new guy as well.
#53: Fatigued, exhausted, and resentful, he wanted to prove that he would do anything it took to win. He was sick of war, sick of the responsibility and stress, and just wanted it over. It was more of a desperation move than anything.
Hope that helps!