I find this site useful: Urban Dictionary (http://www.urbandictionary.com/define.php?term=Bunny%20Killer)

Based on this definition, does it make sense in your case?
WallyH

I suspect he might have meant 'bunny boiler'. A reference to an old flim, where the woman became obsessed by the man she had been seeing and stalked him, killing his pet rabbit and boiling it. Generally used to refer to women who are obsessive in their relationships and won't give the guy any space.

Is the girl pregnant? The old term "bunny killer" can refer to a woman who is pregnant as it used to be thought that the urine from a pregnant woman was fed to a rabbit. If the rabbit died the woman was pregnant.