Why are Malcolm and Macduff considered to be heroes in Macbeth?

Macbeth takes the throne by murdering the rightful king, Duncan, and upsets the natural order of things in Scotland. At the time, a king's rule was considered to be in accordance with God's will, so the murder of Duncan can be viewed as a rebellion against God's will. By taking the throne unlawfully, Macbeth throws Scotland into chaos. 
The two most prominent characters in the play who are resolved to restore order in the country are Macduff and Malcolm. Macduff is among the first to doubt the legitimacy of Macbeth's claim to the throne. He senses that Macbeth gained power by resorting to illegal means, and, of course, he is right. Macbeth murders Malcolm's father, Duncan, as well as Macduff's family. Both Malcolm and Macduff have two reasons to attempt to depose Macbeth. First, they want to restore peace and order in Scotland and make sure the rightful ruler takes the throne. Second, they are motivated to depose Macbeth because they want to exact revenge on him for killing all the innocent people, including their own family.
Malcolm joins Macduff in his quest to depose Macbeth, and they both succeed in defeating him and his army. They are heroes because they manage to save their country from the tyrannical rule of a cold-blooded murderer.