How do you kill a cow killer?

I really hope you are asking about a "cow-killer ant" and just just a "cow killer." If you're asking about the former, then try the following: Step on it. Spray it with Raid or some form of wasp killer.

If you're asking about the latter, that's something which is condoned on this website, since it is an illegal act of violence to kill another human, even if they make a living killing cows at a slaughter house.