Aggression in a dog comes from various areas. Many times aggression stems from fear. Some dogs are just inherently mean. Others have not learned to behave socially with people or with other dogs yet? While I advocate working with dogs and trying to stop and correct aggressive behavior there are instances where you just won't get this out of a dog. I would hope in most cases aggression can be corrected, however in those where it will not go away often the options are limited. I do not believe in giving up on a dog, but I have seen many that will bite their owners, friends or just anyone sometimes unprovoked? In these cases I believe that a dog must be dealt with, unfortunately sometimes this means the worst possible scenario.
Where fear is the cause of aggression I am one that believes in most cases you can slowly and steadily socialize and work with a dog no matter its age and get it to where it will come around? This process requires a lot of patience and a lot of work. Sometimes an enormous amount of socializing and time spent putting the dog in various situations while you supervise and watch the dog closely. Many dogs once they overcome their fear they become a joy to be with. Dogs and people suddenly become their friends and they experience a whole new world for them.
Remember when working with an aggressive dog it is important to seek a professional in the industry someone that can and will be able to help and or render a decision on a dogs behavior and the likely hood of that dog overcoming their aggressive behavior.