Does MacBeth really have to be a tragedy?

I mean why couldnt Shakespeare have made it a commedy? Why couldnt Lady MacBeth have been happy with there new title? Why did those stupid witches have to show up and ruin everything? I mean come on really..... it annoys me that MacBeth would have never killed Duncan if Lady MacBeth hadnt shoved the knife right in his hands and practically forced him to do it. I mean I feel somewhat bad for MacBeth hes a coward who cant stand up to his wife but hes a warrior who kills hundreds of men on the battle feild.....I guess I actaully like his character until he has Banquo murderd....than I loose alot of respect for him........So I just had to get that out there its been bothering me <.<