Was Macbeth a good man turned evil or was he evil all along?
This is the ultimate Catch-22 question. Every one of us has the potential to do good or evil deeds. It is up to us to use our free will to make proper judgments as we see fit for each experience we encounter.
Having said that, Macbeth knows the witches' prophecies will come true. Two of the three are true when he decides to be persuaded by his wife to kill Duncan. Part of his argument against killing Duncan is that he can wait for Fate to bring him the crown, yet he doesn't do that. He hears that Duncan has bestowed this honor on Malcolm, and he loses his mind. The witches plant a seed...the same seed...in both Macbeth and Banquo's mind. Only Macbeth acts evilly on the information.
The potential for evil was there already. It depended on Macbeth's choices--he chose to be swayed by the easy road to success than to wait for the right and honorable way to attain the third and final title in the prophecy: KING.
The evidence from the play (pre-prophecies) shows that Macbeth is quite a well-respected warrior/soldier/general. He is about to be handed a title (Thane of Cawdor) because of his brilliant deeds on the battlefield. For Macbeth, prior to hearing the prophecies, life is humming along quite nicely. Then, the seed is planted...and it grows. Macbeth does not turn away from it. The question you ask is a subjective one, but one that can still be answered with a hint of surity based on evidence from the text.
These are questions that are merely judgement calls for the reader. Did he suffer a mental illness that finally surfaced when the drive and pressure for power reached a breaking point? Or had he always had dark thoughts that he finally unleashed?
My personal opinion is that he snapped. His greed for power finally took over his soul, and once he started down that path, he sank into madness.