[Gpdd] [BEHAVIOR] - Very aggressive guinea pig - has anyone ever seen or heard anything like this?

[a.mills at portset.net]

Yesterday I was at a local pet shop and was looking at the small pets.  The owner of the pet shop was just changing the bedding and feeding the guinea pigs and was picking them up.  She let me have a look at them and we got talking about guinea pigs.  She breeds a few guinea pigs herself and she said that she recently had to have her male peruvian guinea pig put to sleep because he was so aggressive.  She first noticed that he was getting quite aggressive with the females she was trying to breed him with.  Then he started biting people.  Apparently the problem got progressively worse until it got to the stage where he was even trying to attack when anyone tried to feed him or do anything in the cage.  Naturally she did not want to continue to breed from him and handling and dealing with him was becoming impossible so he was put to sleep.  I know guinea pigs can become a bit aggressive if they are fighting and you try and stop them but I have never heard of a guinea pig being!
 this aggressive.  She said she thinks he must have been in-bred and this was what caused the aggression problem.  I also suggested that perhaps he had something wrong with him such as a brain tumour.  I always think of guinea pigs as one of the least aggressive of animals and just find this behaviour really hard to believe (not that I'm doubting what happened, I just can't really see a guinea pig being like this).  Has anyone else come across a really bad tempered guinea pig such as this?  I'm certainly glad I've never come across one.