We can only make assumptions on Macbeth's true state of mind before the killings because we do not have much time before the witches prophecies to observe his manner. But based on the information that he was a general in the army and had good friends like Banquo, it can be seen that he at least appeared to be stable and capable. He enjoyed recognition for his success on the battlefield, and this was most likely not the only time this has happened. To maintain this level of success, he would have had to function in a stable fashion.
After the killing, Macbeth loses any semblance of control and stability. Guilt causes him to be sleepless, and panic causes him to continue his murdering habits. He clearly is not capable of rational thought, as is evidenced by his killing of Macduff's family.
His downward spiral began with the witches prophecy, yet, it could be argued that he always harbored his dark side, and had just been able to keep it in check. It seems debatable that Lady Macbeth was able to turn him into the man he became. Yes, she made the decision that Duncan must die, but he had to have wanted that as well to go along with it.