What exactly are the "signs of aggression" you mention? The reason I ask is because I have 2 Border Collies, a bitch and a dog. The dog is prone to occasional growling but he is as soft as a marshmallow, we've had him since a puppy, 8 weeks old, he has been fully socialised, he has never bitten anyone, or any other animal, nor tried to, and he has been castrated since a year old.
His growling usually occurs if and when we try to take his ball from him - he is almost obsessed with chasing balls. We have succeeded in getting the ball from his mouth and he has never tried to bite us. Therefore, he growls but has never in five and a half years done anything other than that. We never "wind him up" to cause him to growl.
I sometimes wonder why those who advocate having dogs put to sleep would advise me to do the same because one of mine growls, as described? I get the very strong impression that some people automatically assume a growling dog must be categorised as dangerous which, as I've outlined, is simply not always the case, although I'm not naive enough to claim that such behaviour cannot be classed as possibly alarming to someone who did not know better?
I've always believed that owners shape a dog's character in 99% of cases, although there's always a risk of a "rotten apple" here and there. Some people claim that Border Collies ought not be kept as pets because they are brilliant as working dogs. I do not subscribe to that and I say that the key is to try to keep them occupied as much as possible. Perhaps your parents couldn't do that? (You hint at that). Maybe the dog simply needs more socialisation and stimulation?
It's a very difficult quandary, but unless he is truly a danger to humans, which is sometimes really difficult to ascertain, then he deserves a second chance. There are plenty of people with time on their hands who may be able to pay him the attention he probably needs