Satan is in rebellion against the One who is All-Good, he hates what is good and loves what is evil. Is this insane? You bet! Is it the ultimate sin? Yes. Did he choose to be utterly sinful and thus utterly insane? Yes. Are there forms of human insanity that are not the result of a choice? Yes. But all forms of insanity like all forms of sickness are in the world as a result of sin, and the sin that is in the world is suffered by the innocent as well as the guilty. Can forms of human insanity sometimes result from personal choice, yes, I believe it can. Reason and thus sanity is a good and a gift from God, the Light of Reason is never found among those who are wicked and depraved, but since they still have rational souls, they retain a kind of reason that would be cleverness or cunning.
Rule of thumb when it comes to questions of this sort: the gift of a free will means that we can choose to love God or to hate Him, but we do not get to choose the consequences. Freedom doesn’t mean control. If you choose evil you will get evil, you can’t choose evil and then by force of your will make it turn good. You don’t get to choose what is good or what is evil, you get to choose between them. It isn’t really much of a choice, but you do have to make it.