By trying to achieve his revenge on the man who had murdered his father (who was also the Don’s best friend) he had to take the life of his own best friend.
I have mixed feelings on Nero killing Avilio at the very end. I thought it wrapped up very nicely with them finding a mutual understanding since Nero's family killed Avilio's and Avilio's killed Nero's. But I can see where they might have been going with that because I…