Of course, you know that what you are asking is pure subjective supposition. There is no "right" answer to a question such as this, since the play, as it is written is all that we have. It should be made clear that Shakespeare set out to write a Tragedy here, and by the classical definition, the hero of the tragedy must fall from a great height due to his own flawed perception of the world -- his tragic flaw.
For Macbeth, this flaw is his ambition, and it is in place in his nature whether he meets three witches and has a pushy, power-hungry wife or not. Shakespeare makes sure that the speech in which Macbeth actually commits to his course of action, his soliloquy in Act I, scene vii, has him alone onstage. He considers all the very good reasons that it is a bad idea to kill Duncan and then he decides to do it anyway. His reason:
...I have no spur
To prick the sides of my intent, by only
and it is his ambition that drives him throughout the play. Shakespeare has also show how Macbeth both moves away from his wife's influence and even decides to kill Macduff (though Macduff eludes this) even though the witches' apparitions seem to suggest that Macduff is not a threat. Both of these actions show how he picks up more and more steam for his own ambitious, murderous behaviour regardless of those around him.
So, no, I do not think that the character Macbeth would act in any other way. If he did, we have a mildly pleasant story, but not the great Tragedy that Macbeth is. It is his fall, his all-too human hubris, that the audience is meant to witness and pity. Without this course of action, there is no Tragedy of Macbeth.
As for his being a great warrior. Shakespeare goes to some trouble, at the opening of the play, to have Duncan reward his actions in the most recent battle. He praises Macbeth's skills as a warrior:
O worthiest cousin!
....Would thou hadst less deserved,
That the proportion both of thanks and payment
Might have been mine! Only I have left to say
More is due than more than all can pay.
It is important also to the Tragic form of the play that Macbeth begin in this very noble and rather exalted position. This gives him the opportunity, through his own behaviour, to fall from a great place. He, himself, is not a king at the beginning of the play, but like Othello, is a great warrior, praised and decorated for his skill and bravery.
I think that a careful reading of the play and an understanding of the classical requirements of a tragic hero will show that Macbeth is already a great warrior and that he must act in the way that he does for the play to fulfill itself as a Tragedy.