> if a dog has such biting problem, why not put a muzzle on it instead of killing it?

euthanizing is generally the last resort if the dog apart from is also seriously ill(as go figure most fo the dogs that do this werent being properly taken care of), many dogs that were previously involved in fighting can be rehabilitated(and often end up as service dogs even).

but ultimately the problem isnt the dog, they were just being dogs, its the owners that either thru abuse , neglect or both failed ot train and care of them properly and should be held liable for their actions.