An argument could be made that Macbeth also murdered Young Siward because Macbeth did kill him, but it was in the final battle at Macbeth's castle.  Also, when Macduff hears about the murder of his wife and family, he says, "All my pretty chickens?  Did you say 'all'?" (Act 4, sc. 3) which would indicate there was more than just the son that we see earlier in Act 4.

Let's see...King Duncan, Banquo, Lady Macduff, Lady Macduff's son...that would make four murders.  And yes, Macbeth is responsible for them all, unless one wanted to somehow blame the witches and Lady Macbeth for setting him onto the course of action that he chose.  Ultimately, though, he had free will and could have stopped at any time, or not started with the first murder (King Duncan) that led to everything else.