There are definitely several reasons why Malcolm fears and distrusts Macduff that Malcolm states in his speeches found in lines 10-31 of Act 4, Scene 3. More specifically, Malcolm fears that Macduff has been sent by Macbeth to murder him because Malcolm remains an obstacle to Macbeth's abilities to keep the crown.

One reason is stated in lines 14-16. Malcolm points out that they once believed Macbeth to be an honest man; more importantly, Macduff once "loved [Macbeth] well" (15). Furthermore, Malcolm argues that Macbeth has not yet hurt Macduff personally, since this is prior to their knowledge that Macbeth has ordered Macduff's family to be murdered. Malcolm sees Macduff's love for Macbeth and the fact that he bears no grudge against Macbeth as an excuse for Macduff to feel inclined to do Macbeth's bidding. Macduff may have been sent by Macbeth to kill Malcolm, like a "weak, poor innocent lamb" (18).

When Macduff argues that he is no traitor, Malcolm's next argument is that, even though Macduff appears to be good, he cannot understand why Macduff has come to England to help him; plus, even the "brightest" angels have fallen from grace. Hence, Malcolm's essential argument for this fraction of his speech is to say that appearances can be deceiving and that even the appearance of goodness can't be trusted.

A final reason why Malcolm distrusts Macduff is because he sees no motive for Macduff leaving his family in a vulnerable position to come and help Malcolm, unless of course he was commanded to do so by Macbeth. He can't imagine that Macduff would have less of a motive to remain with his wife and child than he would have to come and help Malcolm.

Hence, all in all, Malcolm simply feels it is too good to be true that Macduff would leave everything behind, leaving his family so vulnerable that Macbeth could send someone to murder them, all for the sake of helping Malcolm regain the crown. Instead, he fears that Macduff must be being loyal to Macbeth and must have been sent by Macbeth to murder Malcolm.