In Macbeth, why does Macduff refuse to attend Macbeth's coronation?

Macduff refuses to go to Macbeth's coronation, because
Macduff is suspicious of Macbeth.  Macduff is the only person to question why Macbeth
killed the two guards.  They are witnesses to the crime, and information could have been
gotten from them.  Macduff shows his suspicion by questioning Macbeth.  Macduff knows
something isn't right.  Because Macduff is suspicious of how Macbeth attained the
throne, he doesn't want to honor Macbeth at the coronation.
 

I also think that Macduff went back to his home in order
to reduce his own danger from Macbeth.  If Macbeth was willing to kill a king, then for
sure he would be willing to kill Macduff just to keep him quiet.  Macduff can't spread
his suspicion around, if he is dead.  Macduff knows that he is a potential target, so he
leaves.