I absolutely agree that LT Haverson executing the Huragok "prisoner" on Ascendant Justice was awfully… well, awful. A Covenant non-combatant can be contained and even more so on one of their own vessels. Yet, I'm not shocked as you are by his comrade's general reaction to the execution. For them it's likely to be business as usual to execute Covenant prisoners of war who are uncooperative and combative, or for whom they can't provide basic shelter, sustenance, and healthcare. Conduct of the Office of Naval Intelligence thus far proves Haverson's actions were well in-keeping with their hyper-violent attitudes and strict xenophobia. During the Human-Covenant War, almost acting as political officers (which they do), this spreads throughout the United Nations Space Command through propaganda efforts by Sections Zero and Two.

A question must be raised regarding the Geneva Conventions and its application within the Unified Earth Government. It's not an adoption by the United Nations but merely the world order at the time and continued on by state actors as law today. The documents in its original language regularly uses terms such as "humanely" and "humanitarian", as well as "person(s)." It's common knowledge that Humans at this point in the war do not regard any species or individuals in Covenant civilization as people. That's easy bait for the UEG Senate and the UNSC Security Council to circumvent its adaptation with alien species.

Now the Universal Declaration of Human Rights, through a later bill, and the Mortal Dictata are binding laws in the United Nations. Still, the Covenant aren't Human and any lawful specificity can be reinterpreted as only permissible for the wellbeing of humans. It could even be denied to the Outer Colonies since the UN in theory doesn't have authority over them. Ultimately, the Unified Earth Government seems to have entirely abandoned or refined and amended many 20th century treatises with then-modern thinking.

As for ENS Dowski she honestly had it coming. I'm not saying CAPT Keyes was entirely correct in the minutiae of his conduct but he wasn't entirely in the wrong. Pillar of Autumn was detached from national, colonial, or fleet command authority and Keyes had no hope of reprieve. ENS Dowski violated multiple articles of what may be a refined adaptation of the U.S. Uniformed Code of Military Justice, ones that are directly antithetical to conduct in the face of enemy forces. It was well beyond common knowledge this late in the war that the Covenant only takes Human prisoners as means to an end and will execute them thereafter, regardless of non-hostile conduct or not. Keyes and the crew of Autumn intend to escape Alpha Halo, and failing that, wage a guerilla campaign against the Fleet of Particular Justice. There was no room for hesitation.

The good captain's treatment of his subordinate was not ideal but neither were their circumstances. It just sucks. After the fall of Reach the Fleet was on its last legs and there was little room for error. Captain Keyes made an emotional call with Dowski and it shows, repeatedly. For the safety of his command and Autumn's ships' company Keyes had to pacify ENS Dowski. I am shocked he didn't put the frankly insubordinate and absolutely senseless ensign out of her misery and summarily execute her. Yet, both Keyes and Dowski, and indeed much of the crew, were emotionally distraught with the loss of Reach and this immensely daunting alien setting. That's a result of continued, almost willing misunderstanding of mental health, physical stressors, and so forth and their complexities in combat.

So, to sum up my response, imperialism and fascism is a bitch.