Was Macbeth insane the whole time?we don't really get that much information about him from before the beginning of the play
Macbeth and insanity lies within your interpretation of the play and providing relevant examples. Macbeth's tragic flaw is ambition and that leads him to commit extreme and cruel acts. From the beginning we know that the weird sisters planted the seed of ambition in his mind. Whether it really was destined for Macbeth to be king is left for the reader to decide. What we know is that the weird sisters put the idea of fate into work by showing Macbeth a glimpse of a future, a future that may or may not have happened. However, by putting the idea into his mind in the first place, Macbeth drives the action which leads him to murder King Duncan, and various others because he is under the belief that he was meant to be king.
Had Macbeth chosen not to act and believe that fate would work him up to king either way, he could have spared the lives of many and achieved his destiny in a good way. Macbeth, however, chose to commit the acts he did. It is up to your interpretation to decie whether he went insane with the idea of power, was just overly ambitious and ruthless, or was insane from the beginning.
It is also smart to consider Lady Macbeth's role in driving her husband's actions. Ambition combined with pressure from loved ones can be a very powerful thing and not necessarily require insanity to commit insanely unthinkable acts.