Well, there's a problem, because we can't make full sense of Malcolm's character without the final act. That's when we see that Malcolm could be a good king, and that he has the mettle to rule well.
That said, we can get a lot about him from earlier in the play. His first speech (in Act I, scene 2), shows that can and does recognize heroic acts done by others. His report on the Thane of Cawdor's death shows that he can evaluate men well, as well as virtuous action.
That said, he seems a bit of a coward when Duncan is killed. His first lines after that show an instinct for self-preservation, not revenge or justice.