No. Genocide takes place against an ethnic group or a specific nationality for example. I am not sure what the defense would be. Are you going to say that since some Serbs killed Bosnians, now the Bosnians in self-defence can wipe-out the Serbs off this planet? Or if Bosnians attacked Serbs, would the Serbs have a free hand in killing-off all the Bosnians on this planet?
There is no defence for genocide; it is a brutal and atrocious act(s) of violence!
"The truth is rarely pure and never simple." Oscar Wilde