Was Macbeth truly new to the profession of killing in act 3? If so, then how?
Macbeth is a warrior. He is not new to killing in order to protect his allegiances--his King, his fellow warriors, and the land he loves. In Act III, he DOES plot to kill on his own, however. Without any prompting from the witches (directly, anyway) or Lady Macbeth, Macbeth makes the fatal decision to hire murderers to kill both Banquo and Fleance in order to secure his path to the throne. This is the path of no return, which is reiterated by the appearance of the ghost of Banquo at the dinner and the scene of madness in which Lady Macbeth herself misreads his guilt and also defends him to the nobles with a flimsy excuse of a mental anxiety suffered since childhood. The murder plan involving Banquo and his son also makes it easier in Act IV for Macbeth to send murders to MacDuff's castle and slaughter everyone there. Once Macbeth makes the choice to murder for his own advancement (not for defense of country or under the influence of his evil wife), then his character takes a dramatic turn for the worse and nothing he does to redeem himself can rescue him from the gruesome resolution of the play.