Why did Shri Krushna Kill a Cow?

Many non-Hindus claim that Shri Krushna has also killed cow!  What is wrong if they do the same?
They conveniently forget to mention that Shri Krushna only killed Dhenukasur because it was actually an asur (demon) pretending to be a cow!

 

Shri Krushna has killed cow, horse, crane, python etc when they were actually an asur pretending to be that animal.  Like the proverbial 'wolf in sheep's clothing', if an evil person pretends to be good, should we refrain from punishing him simply because of his outer appearance?  By pretending to be a cow or a horse or a crane, asurus committed terrible crimes and assumed people would spare them because of their appearance.  They used the gullibility of the masses to carry out their nefarious activities for many years.  Shri Krushna put a stop to that and exposed their true characteristics. 

 

Shri Krushna even asked Arjun to kill his own cousins, grand-uncle and even guru because they were perpetrating and supporting evil.  To do justice, sometimes you have to take unpopular decisions and Shri Krushna always delivered justice - true justice.  He punished the people who committed the crime as well as those who supported and instigated the crime.