What puzzles me is why he tried to kill Duncan.

Someone tried to kill Duncan earlier this evening - merasha on the dagger.

The town double-crossed the three gunfighters after they killed Duncan, and the men are expected to seek vengeance.

If Macbeth hadn't met the witches, would he have killed Duncan?

He attempts to use the armor to kill Duncan as revenge.

It will do me and my wife no good to have ye killed, Duncan.

Perhaps he didn't intend the drug to kill Duncan.

Or did he act on his own free will intending to kill Duncan all along?

Police accepted the fact that a hitchhiker had killed Duncan at the spot where his body was found.

Towards the end of the second book, she nearly killed Duncan when she rammed her car into him.