Did you really just ast why we dont [miss-spe[[insert bad word here]]lled word] Kill the Covenant? Uhhh, because they dont exist yet, but if they did, then trust me, 95% of the world would be trained on how to kill them by now.
Ohh, you ment in the story!? well because we dont have the balls to shoot MAC cannons at our own homeworld. Think of all that non-radio active weapons they could have killed them with. Missles, Spartans, Drop pods, MAC rounds, More MISSLEs, rubble, the French... no wait, We need them to lead the Ristance after they take over "EUROPE", never mind. But do you see what I mean, That would be like Five Billion people dieing from the Crap falling out of space, Thats like(right now) 98% of the worlds population. No one man could cause that much murder these days, Now IF they cloned Hitler, then He(even his clone!!) would have no problem with that.