Agressive behavior

Question
My boyfriend saw this dog roaming our neighborhood and Tom, myself and our neighbor played with the dog and kept it company.  The dog is around a year to year and a half years old and a pitbull/german shephard mix.  We called the animal shelter to come pick it up and when they came they said if no one claimed the dog or wasn't adopted they would have to put it down within 6 days.  We felt bad because the officer also said it maybe hard to adopt due to being part pitbull.  We agreed to keep it for a few days to see if they owned came looking for him.  Also, the dog had not been fixed.  I bought him a few treats and a couple of toys and a made a makeshift bed in the garage.  He seemed very content and happy to be at our house and started to respond to commands we gave him.  We had him tied up to our workout bench since we knew he would take off if we didn't.  The next morning came and all as good until the neighbor that was at our house the day before petting the dog came down our driveway to check on the dog.  Thank god the god was tied up since he went after him as he was going to attack him and tried to bite him but he was far enough away and the cord didn't allow him to reach him.  We couldn't believe that the dog was that territorial and decided to call the animal shelter back out and take the dog since he would be a liabilty and we didn't want him to attack anyone who came close to our house.  Is that normal behavior for a dog that we had just met or was this dog already trained to attack?  Just curious for future dogs we may choose on our own.

 Thanks,
  Rita

Answer
Actually, most Pits are people friendly.  Some dogs are territorial, but my guess is that the dog you had may have been barrier aggressive - meaning that he knew he was tied, and that made him fearful and wanting to use a "best defense is a good offense" strategy.  Of course, not having seen his behavior, I can't really be sure, so it's just a possibility.  Given that you reported this as aggression, the likelihood is that the dog is no longer alive, and to be honest, it's unlikely that the average pet home could deal well with a barrier aggressive Pit anyway.  So sad.  These are brilliant working dogs that deserve a better life than what they usually get.