While the argument on Macbeth's accountability of his actions generally depends on different moral perspectives, you can take into consideration the two main positions in this matter. Even if he believed in the prophecy of him becoming King, he chose to kill [current] King Duncan instead of waiting for his passing to fulfill this prophecy. His best friend started noticing Macbeth's off behavior and reacting to it, but Macbeth interpreted it as Banquo being after him and had him killed too. Because of this, some people argue that Macbeth is deserving of mercy and pity due to the evidences of his onset insanity/dementia, and growing paranoia and anxiety (present in Shakespeare's development of Macbeth), hence not able to make righteous decisions, even if he wanted to.
However, as horrible as these acts were, they were also calculated and rational. This is the main argument to hold Macbeth accountable for all of his actions and provide proper and extreme punishment.
