some dogs are just like that, despite whatever "nature" their breed is supposed to have.
i owned a labrador, who was fixed, that went absolutely skitzo around other dogs. he just wanted to fight all the time. we couldn't walk him. he'd escape from our yard [the shit was smart. he'd always figure out a new way out] and go tearing off round the neighbourhood.
the last straw was when my dad walked him for the last time. he was the only one who could. some idiot had his little toy dog off the leash walking around the street and it headed straight for my lab, who proceeded to pick up in him mouth and shake him. he nearly killed it. the only reason he even dropped the poor thing was because my dad kicked our dog in the stomach. we had to get him put down after that. he was too dangerous and we didn't want him doing that to a child one day, despite how lovely he was around people. training him just didn't do anything and my dad wasn't about to pay for specialist training [in our house the dog is just a dog and we don't pay anything more than to keep it fed. if it gets too sick, it gets put down].
if you can be bothered spending the money for training and being consistent with him [as consistency is the key] then he could be alright. my dad just can't be bothered doing any of that. if a dog's aggressive it can be difficult to change the behaviour if you're not keeping up the training all the time.