Eventually, the family figures out the truth and tracks down the doctor in his filthy animal murder den. There, they discover that he's not only a crooked vet, but also a gun-wielding vet, as he demonstrates when he very nearly murders the dad. He's only stopped when the dog from Frasier bites him in the penis.
That's violent, but also hilarious and heartwarming, so it's fine. Rather less heartwarming is when the kids crash the family station wagon into the lab, hitting the table full of sharp, precariously positioned syringes ...