Was macbeth a tragic hero led astray by the witches' predictions or simply a greedy, ambitious killer?

Definitely the latter. A good person would not have killed for his own ends. He was told his friend's children would be kings, not him. The witches didn't tell him to harm anyone.

You could argue either way if Macbeth was a tragic hero blinded by the wicthes or if it was greed that blinded macbeth into killing all of those people. in the end its how you viewed macbeth in the play and what you think happended to him through out the play.