Those two events are in relationship with the rebellion formed while Macbeth's tyrant reign.

After being crowned King, Macbeth begans to see in every single person someone who can put an end to his reign, so wants to eliminate the principals ones.

Banquo was starting to suspect that Macbeth got ambitious and killed the king to take his place, while remembering the prophesy told by the weird witches.

And also the reason that he killed Macduff's familly is that he had a very high place inside the rebellion, so he wanted to kill him to destroy a big part of this rebellion. But he wasn't at his house that day, so the murderers only killed his familly.

But even if Macbeth didn't killed Macduff, his victim still suffered much.
And Banquo's murder will continue to haunt it for a long time (as a ghost)