In Shakespeare's tragic play Macbeth, Malcolm and Donalbain flee Scotland in fear of their own lives. Unfortunately for them, this could bring suspicion upon them regarding their father's, King Duncan, death.
Macduff states to Ross, after finding out about Malcolm and Donalbain's escape from Scotland, that Malcolm and Donalbain
Malcolm and Donalbain, the king's two sons,
Are stol'n away and fled; which puts upon them
Suspicion of the deed (II,iv, 24-27).
Macduff, while stating openly that he believes Malcolm and Donalbain were responsible for their father's death, undoubtedly does not truly believe it.
Macduff and Ross go on to discuss how nature is in upheaval. Ross states that Macbeth will certainly be named king (indirectly stating that only in a place where nature is at not at ease could Macbeth be named king). Macduff tells Ross that Macbeth had already been named to the title.
The truth behind Macduff's beliefs regarding Duncan's murder is apparent later in Act II, scene iv when he states:
Lest our old robes sit easier than our new!
Here, one can illustrate the fact that Macduff is not keen about Macbeth being king. Instead, Macduff is admitting that he realizes Macbeth was responsible for the murder of Duncan and, he is simply not wanting to admit openly that he knows.
The fact is that when the father is murdered, the sons are expected to be at the side of the slain father and not run away to save their own lives. In that case, anyone can place the needle of suspicion on them. However, Macduff, being a smart and intelligent man, as is displayed later in the play, may not consider Malcolm and Donaldbain to be the murderers because the murder took place in Macbeth's castle, and without Macbeth's support the crime cannot be possibly committed. Besides, both are young. Another reason that absolves them of the crime is that Malcolm has been named the Prince of Cumberland.