How far can Macbeth be viewed as a coward when he had to kill Duncan?

Macbeth can be perceived as a coward for succumbing to his wife's demands and refusing to maintain his integrity when she aggressively ridicules him for hesitating to follow through with their foolproof plan. In act 1, scene 7, Macbeth tells his wife that they cannot go through with the plan because Duncan is an honorable, benevolent king and their deed will surely come back to haunt them. Lady Macbeth responds by calling him a coward and expressing her disgust at his frightened, uncertain demeanor. Instead of challenging his wife, Macbeth succumbs to her wishes and agrees to commit the murder against his will.
In act 2, scene 2, Macbeth follows through with the murder but exits Duncan's chamber a changed man. The fact that Macbeth murdered the defenseless king in his sleep is also a cowardly act. Macbeth once again reveals his cowardice by refusing to return to Duncan's chamber to place the daggers by the deceased chamberlains and vividly describing his hallucinations. Macbeth’s hallucinations are a result of his fear and guilt. After exiting Duncan’s chamber, Macbeth swears that the servants said, "Sleep no more! Macbeth does murder sleep" (Shakespeare, 2.2.35-36). Macbeth also displays his cowardice by immediately regretting his actions and struggling to control his emotions. His wife ends up returning the daggers to Duncan’s chamber and has to calm him down.

Macbeth did not have to kill Duncan and, in fact, struggles with the moral issues of killing a good king, one who has been personally generous to him. However, Macbeth can be seen as a coward for two reasons.
First, he has already determined he will not kill Duncan when his wife gets hold of him. She insists he must do it because he said he would and states very strongly that he is not a real man if he does not do the deed. She says she would do it if she were a man and would even dash her baby's brains out if she had promised to do so. Macbeth is a coward in that he is afraid to stand up to his wife and afraid to look bad in her eyes.
Second, he kills Duncan while Duncan is sleeping and defenseless. This is not a fair fight. Worse, given the social expectations of that time, he is supposed to be offering Duncan hospitality by inviting him to his castle. This would include protecting him, not murdering him in an underhanded, cowardly way.