Okay...
1. You say that Sigma was sane in X8. He was not. Every time Siggy transferred himself to a new body, he got crazier. Compare X1's Original Sigma to X6's Reborn Sigma.
I am only going to address this one due to the -popular- misconception of what the term "psychosis" refers to as well as X mislabeling Sigma an unimaginable amount of times, as well as fans of the series. (Psychosis = insanity)
Insanity does not by any means refer to the act of taking lives outside of the laws or standards of a certain living period. It means that an action is taken from a deep misunderstanding of what truly is and reacting off the misunderstanding/ignorance without your mind going into deeper detail of your actions/caring to the least.
In fact. Here's a copy and paste of the very definition of insanity.
1: a deranged state of the mind usually occurring as a specific disorder (as schizophrenia)
From what is shown in Maverick Hunter X, Sigma did not go through anything of the sort as schizophrenia, bi-polar disorder, or show an actual signs of psychosis. Sigma plotted, planned, and when he got the chance, he took time to -listen- and understand from Dr. Cain's words about X. Needless to say, someone who's suffering from insanity would not bother to try and understand anything. They would simply react. From what is shown in X7 the virus doesn't exactly turn you into a raving lunatic as the Hyena showed, he was in extreme pain and was at a loss for logical actions. (I.E, thinking that killing Zero, X, Axl who showed up right when he started suffering would stop it.) And before anything is said, illogical decisions or actions are usually made/done when someone is under unimaginable pain.
2: such unsoundness of mind or lack of understanding as prevents one from having the mental capacity required by law to enter into a particular relationship, status, or transaction or as removes one from criminal or civil responsibility
Sigma understood every law, every nook and cranny, analyzed it and was not satisfied with the current state of the world or popular opinion. His constant debates with Cain are proof of this. He has the mental capacity to understand the law, he just doesn't see why he should be bound by the laws and not create his own utopia. Which, when you think back on modern history, seems to be the way that -every rebel faction in time felt.- Be they on ethical right or wrong.
3 a: extreme folly or unreasonableness b: something utterly foolish or unreasonable
Now, this is the one where proves Sigma is not insane. When you consider the positions Reploids are placed in, being servants for an obviously weaker species, forced to slaughter their own on a single command or order, to work endlessly with seeing so little as enough to keep themselves either functioning until their use is over, upgrading or maintaining weaponry, or just for the satisfaction of their human creators how can you not go: "Okay.. I see where they were going with this."
Be whatever title they were given, they were humanity's slaves in the X series more than companions. Humanity couldn't do half of the labor they put the reploids through, fight the wars they forced the reploids to fight or last as long as their creations could. Needless to say, it'd be completely unbelievable for there to not be rebellions under those conditions. Or even then, understanding why there are rebellions. They've created a race that is self-efficent, self reproducing by using the same machines to make themselves, and powerful that the question of why they're still scrubbing the back fat of Trucker-Joe comes to mind.
Which is where Sigma's rebellion comes in, and by all means, it's logical. Namely under the terms of "Survival of the fittest". And in a world where robots can do all the work? I can just guess humanity is just about 80% body fat at that point. While him leveling a city with missiles is unethical, it's something that is logical in the sense of "sacrifice for the greater good." It sends out a message to your enemies that you are not--and you will not play games with them. That you -will- wage war like they've never seen.
When you consider it, Lumine was very right to say that Sigma was perfectly sane, both logically and literally by means of definition of "insanity". What was the cause of it was the sub-consciously, there was his decision to rebel under those conditions. Which, if you took another race of people and have them bossed around by one race--well, we've all seen that outcome in history.
Insane does not fit the bill for a description, if anything Sigma woud earn the title of being void of all moral ethics. X6 being the only exception due to Gate not being done with repairs. While vague, he could have been unstable due to not being completed.
Edit: And for the sake of making it simple, here's differences in insanity and completely unethical in comics.
Carnage: INSANE. Killing random bystanders because--well, he was literally bored.
Punisher: Unethical. Killing people who are dangerous, and avoiding the very justice he stands by and chased by.
Scarecrow: INSANE. Killing because he gets a kick out of how people react to their deepest fears and nightmares, and jotting down the information for further use.
Joker: Extremely unethical. He is...literally too sane to the point where it effects him in negative ways. Joker causes carnage in an experiment to test humanity in it's darkest moments, revealing--to all--that while they may think they are not capable of something, doing one little action and taking one life will push them into madness. And for all their plans, one random act will shatter them all.
Every time the Joker did something, and people acted how he expected them to act, proved his lack of insanity--under the definition, that he understood the situation, reactions, and causes.
Needless to say, this actions weren't for his own benefit but mainly to test humanity of their claims.
Being a murderer or being a mass murderer does not make you insane.