Malcolm is suspicious of Macduff's plea to help him overthrow Macbeth for the following reasons.
Firstly, Malcolm states that Macbeth was thought to be honest and that Macduff loved him, so he is really not convinced that Macduff has changed:
This tyrant, whose sole name blisters our tongues,
Was once thought honest: you have loved him well.
He hath not touch'd you yet. I am young;
You may deserve of him through me, and wisdom
To offer up a weak poor innocent lamb
To appease an angry god.
The fact that Macbeth has not done anything to Macduff puzzles Malcolm, and he starts wondering if Macduff wants Malcolm to join him against Macbeth only to be lured and killed eventually by the two of them. Malcolm refers to himself as "a weak poor innocent lamb," which means he fears he may be the victim in this affair.
Secondly, he cannot understand why Macduff leaves his family unprotected. If he is truly afraid of Macbeth's evil intentions, he should have not left them behind.
Although Macduff attempts to dispel all these suspicions, Malcolm goes on to test him, saying he is full of vices; he is voluptuous and avaricious and does not possess any of the virtues that kings should boast about. When Malcolm realizes that Macduff is utterly disillusioned and that he bemoans the tragic state Scotland is in, Malcolm becomes convinced that Macduff's change of character is genuine:
Macduff, this noble passion,
Child of integrity, hath from my soul
Wiped the black scruples, reconciled my thoughts
To thy good truth and honour.
Therefore, Malcolm accepts to join Macduff in order to defeat Macbeth and restore peace in Scotland.