Macbeth definitely had a predisposition for violence, but he was a valiant and loyal soldier before the witches entered his life.
There is evidence that Macbeth was once a good man. The sergeant tells Duncan that Macbeth was a big reason for their success in battle.
For brave Macbeth—well he deserves that name—
Disdaining fortune, with his brandish'd steel,
Which smoked with bloody execution,(20)
Like valor's minion carved out his passage
Till he faced the slave… (Act 1, Scene 2)
It certainly sounds like Macbeth was really good at being a soldier. But does that make him a good person? After all, being a soldier allowed him to channel his bloodthirsty or murderous tendencies into acceptable actions. He was allowed to kill, and encouraged to do so. Killing in battle is one thing, but Macbeth actually cut the guy in two down the middle!
Why did the witches choose Macbeth? It might have been because they knew that he was predisposed to violence. After all, he was not the only one that heard the prophecies. Banquo was there too, and he did not become a murderer. In fact, he tried to convince Macbeth to take what the witches told them with a grain of salt—even though they said Banquo’s sons would be king.
But ’tis strange;
And oftentimes, to win us to our harm,
The instruments of darkness tell us truths,
Win us with honest trifles, to betray's(135)
In deepest consequence—
Cousins, a word, I pray you. (Act 1, Scene 3)
It does not take much to spur on Macbeth’s ambition. When he finds out that Malcolm was named Duncan’s successor, he gets angry. He does not listen to Banquo. He seems to think he’s entitled to it because the witches told him.
Lady Macbeth thinks he is a good person, but she does not necessarily consider this a great thing.
Yet do I fear thy nature;
It is too full o’ the milk of human kindness(15)
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act 1, Scene 5)
Lady Macbeth is certainly no teddy bear herself, but she is the best judge of Macbeth's character, since she knows him best. She is actually the one who devised the plan, talked Macbeth into it, and made sure he followed it. If he had not had her to encourage him, he might never have headed down the path to murder.
So was Macbeth evil, or did he become evil? We can't really know for sure. There is evidence to suggest this much: he was predisposed to evil, and he was strogly encouraged.