humane society tucson Adopted a dog that viciously killed our little terrier tucson, Internet

Adopted a female chocolate lab from the THSSA, (Tucson Humane Society of Southrn Arizona) on April 23rd. Returned the dog on 5/4, after she picked up our little terrier, and shook him to death. The dog seemed to adjust to our other pets: pomeranian/pekinese, pitt bull, yorkshire terrier, while we had her in our household.
When we adopted the lab, we brought both Katrina, our female pitt bull, (who had a shock collar on, as she weighs 80#, and likes to be elsewhere) and Guinness, our 5# terrier, to the HSSA for an evaluation which was required, to meet the adoptive dog that day. We spent three hours waiting past the instructed time. My husband and I were interegated by someone "20- something" regarding our dogs and told immediately "we don't like shock collars". We informed her we hired a trainer, and the collar is OK. She stated clearly, "The THSSA does not believe in shock collars" on pets. Guess what? The "Rattlesnake Training" program, which THSSA endorses, uses shock collars. All our pets were taken to the course. Period. We know the trainer.
I  read the adoption contract at the time, which states the responsibilities the new owner has, and
the HSSA has none. We took the dog home, and, she killed our Yorkshire terrier after ten days. The lab seemed OK, but all the pets in the house, just didn't like her. The HSSA emphasizes there's an adjustment with the adoptive pet, and time is needed to allow this. Returning a dog is discouraged.
I listened to the Society's rules on adopting a dog, and didn't listen to my own family pets.
The day little "Guinney" was killed, I intervened when the lab picked him up and shook him, but it was too late. He had a puncture wound in his chest and eventualy stopped breathing.  
The lab was returned to the HSSA and we were later informed, under consideration for transfer to a chocolate lab society. We insisted on euthanization and eventually it was done, as no other family
needs this.
My concern: John Q Public adopts a pet, signs a contract, which states the HSSA is exempt from any problems. This dog was considered to be kept in the system despite a clear problem. 
When is the line drawn for euthanization? What knowledge did the THSSA have on the dog, we weren't told in the first place? The dog was at the Humane Society, for two months, and not adopted prior to our unfortunate encounter.