Castithans are obsessed with cleanliness and order, or at least the appearance of it. Their society is heavily ritualized and rigidly stratified. They believe in everything having a proper place. A common Castithan saying is "Seeming is Being." Outward appearance is very important, and Castithans are very careful to at least appear to be proper. The central ritual of Castithan family life is not the shared meal, as it is with Humans, but the shared bath. The color schemes of Castithan clothing and home decorations involve lots of white and light shades of gray. Even their skin and hair colors are pale white, as if they've been bleached clean.
Irathients, on the other hand, couldn't care less about these things. They are simple and down-to-earth. They live in tune with nature, and because their immune systems are so strong they don't have to worry about cleanliness all that much. As a consequence they always seem dirty or unkempt; their skin and hair are varying shades of brown and red, like dirt. Their culture is less nuclear-family oriented as it is tribal; people who aren't blood relations have very little problem integrating into their groups. They put a greater emphasis on independence and individual desires than they do on familial and/or social obligations.
Little wonder that, of all the Votan races, these two have the most hostility toward one another. They're exact opposites.
It turns out that Nolan "The Butcher of Yosemite" was facing off against Rahm Tak on the Votan side, though neither knew it at the time, during the Pale Wars of approximately twenty years prior. Now that they have been brought back within striking distance of one another, Season 3 could be thought of as a re-run (in miniature) of the Pale Wars. Word of God is that Season 3 will in fact explore some of the history of the Pale Wars in addition.
The re-run of the Pale Wars has essentially come to a close, this time with a similar dramatic act decisively ending things once and for all. In fact, in some ways the terminating incidents are both acts of defiance.
Nolan started out as essentially a rather standard pro-human bigot, which is further backed up by the way he initially rationalizes having an Irathient foster-daughter: Irisa is "special" and unlike the other Votans he's killed. This is not in essentials different from how real-world racists often make mental exceptions for friends or family they have who fit the ethnic category they're prejudiced against.
Since then, however, it's clear Nolan since has regretted his earlier attitudes, realizing they were born of war and privation; he refuses to discuss in detail what he did during the Pale Wars and there are cues in his body language when watching his and Irisa's memories in "History Rhymes" that indicate he is not comfortable with the things his younger self said and did in that era. However it's clear that his way of raising Irisa is still marked by his evolution to his 2040s-era self, since she has a much less rosy view of how he exposed her to casual anti-Votan racism, and taught her to essentially become a weapon.
Through flashbacks, we learn that the scar on Datak's left hand is the result of being disciplined by his strict father, for his pride and his bad temper. This hand, therefore, serves as a constant reminder to him of his conflict with his father and his skewed sense of honor. This is the hand he chops off during his plan to take out Rahm Tak. Symbolically Datak is sacrificing his honor for a greater good. Or, alternatively, he is quite literally severing his ties with his old Castithan life, and living as a man unfettered by honor or sentiment. Either way, Datak will be forever altered.
Nolan's backstory is basically that of a Sci-Fi Geek who consciously lives out the character archetypes he's familiar with when actual aliens show up. A young boy when the Votans first arrive, he is at first wide-eyed and idealistic about meeting new species. Then the Pale Wars happen, he loses loved ones and the world he knew, and he morphs into a hair-triggered, alien-squashing Space Marine (almost literally a typical Video Game Protagonist). After the Pale Wars he softens into a cynical but charming Space Cowboy type; it's so obvious that Berlin calls him out on it, and she's not too far off. And finally, in the Season Three finale, he pilots an alien spacecraft into the unknown, saving the world and bearing witness to all the wonders of the universe. Over the course of his experience with aliens he goes from Jean-Luc Picard, to Corporal Hicks, to Han Solo, to Metaphysical Savior of Humanity.
