Both are key characters
My answer would be as follows:
Banquo and Macbeth are two pivotal characters and I will try to set forth a possible interpretation. At the beginning Macbeth is viewed as a good general and seems to pay the allegiance due to his king. In that he is like Banquo. (a simple and brave general of the king of Scotland) Then there is a strong analogy between the two characters.
But very soon Macbeth reveals another pivotal and powerful aspect of his complex personality: evil. Evil is inherent to his nature and he cannot resist it i.e. he is compelled to yield and fall. Banquo, according to the three sisters, will be the root of many kings whereas Macbeth will be an ephemere king. Furthermore another interesting point can draw our attention: Banquo reappears as a ghost. (This process is called the return of the Uncanny)
This important event highlights the stark reality that the two characters, even if they are enemies, are close to each other. (There is a strange kinship between the two protagonists) The death of Banquo is akin to the death of a martyr whereas the death of Macbeth is only a way out. (Because of the principle of nemesis, Macbeth has to be chastised for his many crimes) In this case there is a kind of counterpoint.
Of course this answer is uncomplete but I hope it will be helpful.