What's your point? As with most issues where you could "swap out 'dog' with 'child' or 'sister' and ask the same question", the opposite answer results. Should it be legal to take your dog to the vet to be euthanized after it bit the baby? Yes. Should it be legal to take your 4 year old child to the doctor to be euthanized because he bit the baby? No.
What's my point?
Did you even read what I was responding to?
If you don't get it, then you didn't read it.
"Should it be legal to take your dog to the vet to be euthanized after it bit the baby? Yes. Should it be legal to take your 4 year old child to the doctor to be euthanized because he bit the baby? No."
You didn't specify whether the 4 year old was rabid or not. ;-)