Why did the SPCA kill Tyson?

Tyson was a giant puppy who, after a puppyhood of abuse and neglect got a chance at happiness, or so I thought, but I was wrong.

Tyson's life videos:

Tyson alive and trusting: http://www.youtube.com/watch?v=v8VDyYOL2MM&feature=related
Tyson is such a good boy: http://www.youtube.com/watch?v=wWOWTn0gIT8