In Macbeth, why are Malcolm and Donalbain suspected of Duncan's murder?

People begin to suspect that Malcolm and Donalbain are guilty of the murder of their father because they flee the country after his body is discovered. The two royal brothers immediately suspect their father's inner circle of the crime, so, realizing that they are in danger too, Malcolm goes to England and Donalbain to Ireland. They hope that their "separated fortune" will keep both of them safe (2.3.163).

Macduff, the Thane of Fife, explains to Ross that people believe that the grooms in Duncan's bedroom did carry out the murder themselves but that they were "suborned" by Duncan's sons to do so (2.4.35). In other words, the common belief now is that Malcolm and Donalbain hired the grooms and paid them to kill their father. Further, Macduff says that they are "stol'n away and fled, which puts upon them / Suspicion of the deed" (2.4.37–38). It is because the two princes have fled that people believe they are behind the murder.

Of course, such a story would suit Macbeth very well, and Lady Macbeth too, as it casts suspicion away from the two of them and onto other people who they would also like to get rid of. We don't know exactly who started this story about Malcolm and Donalbain, but it stands to reason that the Macbeths might have done it, as those who stand to gain the most from the princes' disgrace.

In Macbeth, why do people suspect Malcolm and Donalbain of killing their father?

Suspicion falls on Malcolm and Donalbain for the murder of their father, King Duncan, because they run away after Duncan's body is found. They run, however, because they realize that as Duncan's heirs, their lives, too, are most likely in danger. Donalbain tells his brother they should flee immediately, even before they have had time to mourn their father:

What should be spoken here,

Where our fate, hid in an auger-hole,

May rush, and seize us? Let's away:

Our tears are not yet brewed.

Agreed, they do leave, with Malcom going to England and Donalbain to Ireland. By separating, they are individually safer. As they part, Donalbain sums up their situation:

Where we are
There's daggers in men's smiles; the near in blood,

The nearer bloody.

When they are discovered missing the morning after the murder, guilt immediately shifts to them simply because they have left without explanation. It is Macduff who gives this news to Ross in Act II.

In Macbeth, does Macduff suspect Malcolm and Donalbain for their father's death?

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

were suborned:
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.