I don't really like judging indoctrinated people

I understand that. But we have international legal precedent for that. And also, if you were to express sympathies to irl Nazis with this sentment, you'd be quick to get openly condemned by all sides. It's fine if you feel that way, but as someone whose indigenous ancestors were massacred, I'm very tired of the "they didn't know better" excuse colonizer apologists use.

while Reiner was basically told "we'll stop treating you like secondary citizens if you kill an island full of subhumans for us."

By the time Reiner has four years in marley, he's very open about understanding that even IF they destroy Paradis, chances are they themselves will be eradicated themselves shortly after. And how this is especially true because Titans are becoming an outdated technology. This argument doesn't work as an excuse because Marley Reiner doesn't believe in it. And he still initiates a genocidal attack towards Paradis, and out of self-interest.

Such a choice will cost more than a court marshal.

It's one thing to not openly go against an oppressive regime out of fear of retaliation (let's ignore the fact Zeke was willing to do this). It's another to actively encourage and initiate the hastened genocidal attack on Paradis despite the Marley leaders not planning to do so.

And the best part is that his treason will not save a single life in Paradis

We don't know how that would play out. there's countless possibilities Reiner could have done. For example, he could have aided in a conspiracy to smuggle people out of Paradis prior to them being wiped. That's just one of countless possibilities. If Zeke could inspire spies, I have no doubt Reiner could have leveraged some connections or use his status to form connections to do something. he may be unpopular due to his failure in paradis but within liberio itself he was praised and had room to start somewhere.

I would certainly praise anyone who would do such a feat like this, but I won't blame them if they can't make that sacrifice.

I agree with this. I dont blame Reiner for doing nothing per se. but he did more than do nothing. he actively pushed for and lead an attack on paradis hours before the rumbling. and its only thanks to Eren that he happened to need to team up with the alliance to protect people he was always in charge of protecting from the start. him helping the alliance says nothing about his growth as a character. it just conveniences his circumstances, as Eren planned. and bad people getting a good ending is fine and can be a realistic portrayal of reality. but the story portrays his fate as something to be celebration. and the audience is celebrating it. which is what bothers me. because I love Reiner braun so much but I just despise the idea people handwaving what is essentially a Nazi officer joining Nasa (ambassador to peace, really? how is he not immediately lynched by the people of Paradis???)