if Nero could seen one member of his crew (who is not Kibellah or Marazhai) naked, who would it be?

obviously Conviction doesn't fully exist as an actual system in-universe, but the decisions that he made to get him to his ultimate alignment - were they deliberate products of a conscious system of morality or ethics? Or were they a product of acting on instinct and what felt right at the time?

It would be Heinrix. Nero could have humiliated him by making him be naked and feel vulnerable. At least at the beginning of their relationship. Before they became friends, Nero saw him as nothing more than an Interrogator and torturer. He wasn’t Nero’s personal torturer, but he was still one of them.

Nero follows his own morals, which are based on his experience, beliefs and selfishness. Some of his decisions can be interpreted as merciful acts. For example, when he killed a young rebel from Janus, he saved him from being tortured to death. That is what he believed in. Another example is when in the Adeptus Administratum he didn’t take a ticket from an old man whose daughter had been wrongly imprisoned. In both cases, he remembered his own tragic experience. He showed mercy to the xenos, such as Malice, by releasing him from the Commissar’s cage, remembering that the xenos had saved his life. Malice was obviously pursuing his own goals, but that didn’t matter to Nero, who just wanted to repay the debt. Debts must always be repaid.
No matter how whimsical or twisted his purpose may be, he values the lives and deaths of people and xenos equally. However, some lives are more valuable than others. Nero decides. He allows heretics to board his voidship and occupy the lower decks, which he then turns into hunting grounds to satisfy one xenos. Similarly, he saves Footfall from starvation in order to prove his superiority over Winterscale and win the loyalty of Vladaym Tocara and the Kasballica Mission.
He performs spectacles in which people either see him as an example of faith and loyalty to the Imperium of Man or as a heretic loyal only to the Enemies of Humanity or Chaos Gods. In reality, they are all wrong.

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