In The Tragedy of Macbeth, why might Malcolm be suspicious of Macduff?
The dialog between Malcom and Macduff in the third scene of Act IV shows how dire and dangerous are the conditions in Scotland after Duncan's murder and Macbeth's gaining the crown. Macbeth's tyranny continues unabated, and the forces intent upon unseating him continue to grow. Malcom, the rightful heir to the throne, and Macduff both despise Macbeth. Both are determined to bring him down, but at this point, each is uncertain of the other's motives.
Although Macduff seems to oppose Macbeth, Malcom does not yet trust him. Macduff had loved Macbeth, and although Macbeth has used his power against his enemies, he has not come after Macduff. Malcom says, "He hath not touched you yet." This implies that Macduff may not be Macbeth's enemy.
It also occurs to Malcom that Macduff could betray him to Macbeth in order to secure his own safety and Macbeth's favor. Malcom compares this to offering "a weak, poor, innocent lamb / T' appease an angry god." Malcom apologizes for his doubts; Macduff may indeed be what he appears to be, but Malcom adds that trusting Macduff is difficult because of Macbeth's previous treachery. He was certainly not what he appeared to be.