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