Some rambles on the ending and the codependent nature of Constantin and De Sardet's relationships. Spoilers for the whole game under the cut.
When I played Greedfall for the first time, I couldn’t stop thinking about Constantin’s corruption arc. It didn’t really make sense to me. Why would he want absolute power over the island? He has plenty of power and probably lives quite a luxurious life already. He also says he finds politics boring if De Sardet asks him, so the desire to rule over the whole island full of people seemed to me very out of character.
I do understand that he went mad and everything, but my brain deemed it a boring explanation and kept searching for something more logical.
So here: I think his actions make more sense if they're driven not by greed but by jealousy, less hunger for power and more hunger for absolute love (either platonic or romantic).
Let me explain.
His family is all kinds of messed up. He thinks his father doesn’t care about him, and although we don’t know if he’s actually abusive, I can’t think of any good reason why Constantin would lie about this. His mother, if DLC is to be believed, is responsible for the death of his brother Laurent — Constantin being an heir is clearly more important to her than his wellbeing & mental health after the literal murder of his brother. We don’t know much about Laurent and Constantin’s relationship (although he doesn’t ever mention him & he says nobody cared for him but De Sardet, so maybe it wasn’t good? Maybe Laurent was better suited for politics? That would explain Prince d’Orsay’s disappointment. That’s just my theory though).
Anyway, what I’m saying is that a child needs unconditional love, and judging by this family and Constantin’s line “You've always been the only one to care for me. Our friendship is the only thing that matters to me”, he has only ever received love from De Sardet, which sounds like quite a fertile ground for unhealthy overly attached relationship.
Then the game starts, and the thing is… Well, they are not each other’s everything anymore. De Sardet goes to various cities & talks to every person imaginable & makes new friends, maybe even falls in love. Constantin meanwhile is chained to a palace, especially after his sickness is revealed. De Sardet's world grows beyond him, and he can’t follow them there.
So what do you do when the only person who matters to you grows apart from you? What do you do when you are driven mad with pain and may die alone, and they have to leave you?
Naturally, you find a way to bind them to you for all eternity, and in doing so alienate them from everyone but you. Constantin's smart enough to realize that if De Sardet's friends and allies fought against him, they would turn away if De Sardet joined him. But even if they wouldn't, they're mortal. Their death is inevitable. The only one who can keep De Sardet company for eternity is Constantin. Everyone but him will eventually leave.
That’s why “For you, for us”, that’s why “together, forever”. Not power over the island or some people, but power to keep the only person who loves him unconditionally close to him.
(Again, I acknowledge that this is extremely, extremely unhealthy. That’s exactly what makes their relationship so interesting.)
Anyway, as I said, it feels very in character for him to basically give up on humanity in favor of De Sardet. I can't look at him in the beginning or in the middle and say, “This character would do anything for power”. But I can look at him at any point of the game, any cutscene, and say, “This character would do anything for his loved one”.
