Okay, so lets start from the top.
Marco and Indaria are naturally incompatible.
This is true, but it's so very much not my fault that it's true that it's silly. I view it as a nobody's fault kind of thing, it's just something that happened, and I think you view it similarly? Not sure if there was a point you were driving towards with this section or if you were just setting the stage and showing how this kind of problem was semi-inevitable.
making a character.
Also finishing my sheet:
I wonder, though, if Marco would accept as a solution the immediate surrender and destruction of the chaos amulet, followed by Indaria's complete demotion and placement under the supervision of the ship's ecumenical authorities. If you need an excuse to keep them from defaulting to immediate execution, you can say "I just noticed the tail for the first time!"
It's the only context I can think of where my character would be okay with Indaria staying on the ship, so I thought it might work for you too.
We seem to have a values dissonance problem here.
Lenglon, though... you need to pull your head out, here.
Well that's hostile, guess we aren't going to get to have a polite conversation.
rant about how Indaria ate babies by making a choice that put herself in danger and nobody else.
Yeah, definitely values dissonance.
trying to draw red lines about what's "negotiable" for you is doubling down
The only thing that I labeled as not being negotiable, in my conversation with another player, not the GM, was that she remain a playable character. I drew the red line so far back it's absurd, and the simple fact that I felt the need to draw it at all should tell you how extreme the situation has become, all about a choice that I made a point of staying public about from start to finish in case someone wanted to say, oh, "please don't do that?". I know it's hard for you to understand that asking someone to not do something might influence them into
not doing it, but that's because I'm less of an asshole than you are, and I'll be the first to admit that I'm an asshole.
But besides the question of whether the thing is evil within the game itself, the important part is that you do not get to demand unrestricted freedom of choice, then refuse the same to others. You are being incredibly selfish.
In what world have I demanded anything of the kind in the first place? Unlike you, I abide by a rule of not fucking over friendlies, which, of course, restricts my freedom of choice substantially. You had the freedom to choose for yourself as well. All I'm asking for is that Cael abide by the same, and I have clearly labeled that I'm fine with her getting punished to a reasonable extent and explicitly am fine with it within limits, even though that's not normally a thing a PC should do to another PC.
Values dissonance problem:
So we've got two major aspects of values dissonance going on here. First, there's the simple fact that the 40k universe fosters extremely domineering, judgemental, prejudicial, and hateful behavior. It's all part of the grimdark flavor, which is fine. The problem comes from the fact that this shit is unhealthy as hell when applied to fellow real human beings. Just because the imperium would shoot Indaria on sight, doesn't mean Cael should shoot Lenglon on sight. I think nobody here disagrees with that. However there's some other aspects to it. For example, Macro is the RT is the lord and god of the ship. Cael is
just another player the same as you and me. No better, no worse. Although Marco gets to make demands of Indaria, Cael doesn't get to make demands of Lenglon. Not hard, right?
So take a moment and actually fucking look at what you're saying. You're saying that this choice, made publicly, with zero objection and straight up implied consent of the other players, is a reason to remove another player from the game entirely. You're declaring me a problem player for... taking a risk to my own character, and requesting that I don't be unilaterally executed for it. Yeah, if You and I were in the 40k universe this kind of treatment of other people would be nothing, normal, mild and friendly. This isn't the 40k universe, and people actually do have value here in reality.
Second aspect of values dissonance is the simple question of was Indaria's choice an evil choice? Apparently you say yes it was. The Imperium of Hate would agree with you, 100%, no question. But was it ACTUALLY an evil choice? I think not. It was a risky choice, yes. It was unwise, yes. However, did she put anyone at risk other than herself? No. She did not. She has harmed nobody, she's put nobody at risk other than herself, and I want you to seriously sit down and think for a moment about if it actually was evil at all.
Let me make my bad decision. I, the player (Lenglon), think having Indaria's story being one where she treads on the edge of falling to Chaos but resists actually doing so, will make her have a fun and engaging story. That's my choice. It's my risk to take.
And I'm still waiting for Cael to bring literally anything to the table here. So far he's stated that his hard line is remove Indaria from play permanantly at a minimum, which completely unreasonable and unacceptable, so he's going to have to give, since I've already given quite a lot and don't have a lot more room to work with. I would like to talk this out like functional adults, but a negotiation requires more than one participant, and Cael isn't posting.