It's What Keeps Us Out Of Trouble...

KIRK: How does it work? THERAPIST: Quite simple. Off-on switches, and the large control here changes the strength of the brain neutralizing beam. ADAMS [contemptuously]: Captain, you remind me of the ancient skeptic who demanded of the wise old sage [Hillel the Elder] to be taught all the world's wisdom [the Torah] while standing on one foot. [The substitutions were made for antisemitic Roddenberry reasons; the allusions to Judaism in the Adams-Kirk conflict were explicit in Shimon Wincelberg's script; Adams is implicitly comparing his brainwashing """advances""" to the Torah, himself to Hillel the Elder, and Kirk to the short-sighted goy skeptic for asking reasonable and courteous questions, all of which is wildly fucked-up.] ("Dagger of the Mind")

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SPOCK: Kodos began to separate the colonists. Some would live, be rationed whatever food was left. The remainder would be immediately put to death. Apparently he had his own theories of eugenics. MCCOY: Unfortunately, he wasn't the first. SPOCK: Perhaps not. But he was certainly among the most ruthless, to decide arbitrarily who would survive and who would not, using his own personal standards, and then to implement his decision without mercy. Children watching their parents die. Whole families destroyed. [...]There were nine eye witnesses who survived the massacre, who'd actually seen Kodos with their own eyes. Jim Kirk was one of them.

LENORE: Are you like that, captain? All this power at your command, yet the decisions that you have to make— KIRK: Come from a very human source. LENORE: Are you, captain? Human? KIRK [his manner remaining suspiciously smooth, with no visible reaction to the question]: You can count on it. LENORE: Tell me about the women in your world, Captain. KIRK [still smoothly]: I'd rather talk about you. You must have wanted to perform since you first saw your father act. When was that?

KIRK: What were you twenty years ago? KODOS: Younger, captain. Much younger. KIRK: So was I. But I remember. Let's see if you do. [...] KODOS [forced to read his speech to those slated for death/the genocide]: The revolution is successful, but survival depends on drastic measures. Your continued existence represents a threat to the well-being of society. Your lives means slow death to the more valued members of the colony. Therefore I have no alternative but to sentence you to death. Your execution is so ordered. Signed, Kodos, governor of Tarsus IV. KIRK: I remember the words. I wrote them down.

KIRK: Are you sure? Are you sure you didn't act this role out in front of a captive audience whom you blasted out of existence without mercy? KODOS: I find your use of the word mercy strangely inappropriate, captain. Here you stand, the perfect symbol of our technical society. Mechanized, electronicized, and not very human.

LENORE: Everything's always later. Later. Latest. Too late. Too late, captain. You are like your ship, powerful, and not human. There is no mercy in you. KIRK: If he is Kodos, then I've shown him more mercy than he deserves. And if he isn't, then we'll let you off at Benecia, and no harm done. LENORE: Captain Kirk. Who are you to say what harm was done? KIRK [jerking around, the controlled flirty, mild-mannered mask finally dropping to reveal cold fury in his face and voice]: Who do I have to be?

KODOS: Murder, flight, suicide, madness. I never wanted the blood on my hands ever to stain you. LENORE: I did it for you. I've saved you. KIRK: By killing seven innocent men. LENORE: They weren't innocent! ("The Conscience of the King")

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KIRK: What happened to the Gorn? METRON: I sent him back to his ship. If you like, I shall destroy him for you. KIRK: No. That won't be necessary. We can talk. Maybe reach an agreement. METRON: Very good, captain. There is hope for you. Perhaps in several thousand years, your people and mine shall meet to reach an agreement. You are still half-savage [Kirk gives his usual "fuck you" smile], but there is hope [Kirk sobers]. ("Arena")

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KIRK: I'd like those answers now. First, the purpose of your star flight. KHAN: A new life, a chance to build a world. Other things I doubt you would understand. KIRK: Why? Because I'm not a product of controlled genetics? KHAN: Captain, although your abilities intrigue me, you are quite honestly inferior. Mentally, physically. In fact, I am surprised how little improvement there has been in human evolution. ("Space Seed")

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ANAN [about Kirk's phaser]: I assume that is what you used to destroy disintegration chamber number twelve. KIRK: A very efficient weapon. I'm not afraid of using it. ANAN: My first impression was correct. You are a barbarian.

The deep importance of his own and others' humanity to Kirk isn't simply careless verbiage; it's coming from someone with extensive personal experience of being regarded as intrinsically, genetically subhuman, to a degree that has threatened his life many times. And the racist vitriol directed at Spock is not merely patronizing (though also that), it frames him as not meriting the same kind of dignity and respect for personhood that others do, whether because he's Vulcan, because he's human, or because he's biracial.

That is, there's a recurrent attitude towards him that, because of his genetics and culture, he doesn't wholly merit the dignities afforded to other people (and his life on the Enterprise, at least under Kirk, is actually the least bad of his experiences of this). Even the way people are bigoted towards him often specifically singles out the effects of his genetics on his physiology (and the raw vitriol directed at Spock is heavily concentrated in scenes where Kirk is absent or unconscious). But there's a dehumanizing quality to it, if a subtler one, and Kirk's particular outrage is not only that of a virtuous ally.

I'd also point out that in "The Conscience of the King" in particular, the only person who was not on Tarsus IV yet has a visceral emotional response of horror and disgust appropriate to the reality of it is Spock. He's the only one who really seems to get it without having been there, and engages with it head-on, in an honest, unflinching attempt to truly understand how it's affected the survivors and the danger they're in. Even his horrified phrasing foreshadows Kirk's fury when he finally drops all masks and confronts Kodos.

SPOCK: He was certainly among the most ruthless, to decide arbitrarily who would survive and who would not, using his own personal standards, and then to implement his decision without mercy. Children watching their parents die. Whole families destroyed. Over four thousand people.

KIRK: Are you sure? Are you sure you didn't act this role out in front of a captive audience whom you blasted out of existence without mercy?

KODOS: Some had to die that others might live. You're a man of decision, captain. You ought to understand that. KIRK: All I understand is that four thousand people were needlessly butchered.

Similarly, in "A Taste of Armageddon," Spock understands the horror perpetrated by Anan et al most similarly to Kirk: