In Act 4 Scene 3 of "Macbeth," what evidence is offered regarding Malcolm's fitness to rule?

Malcolm displays several characteristics that would make him a fit ruler.  First of all, he is very cautious with Macduff - Malcolm is not sure at the beginning of the scene if he can trust Macduff, so he proceeds cautiously.  He also displays a love of his country as he mourns over the events that have taken place.  And finally, he displays a wisdom, intelligence, and cunning as he 'tricks' Macduff into revealing that he is indeed trustworthy by pretending to be a terrible person full of vice to see what Macduff's response will be.