If the ONLY way to kill a witch or wizard is to use the Kiling Curse on them, what do you do when they invade your home and mutilate your entire family including your young children? Stand by and watch because you aren't justified the use of the Kiling Curse or Imperio to stop them? That, honestly, just doesn't sound very plausible. In most cases, the law looks to you as justified in kiling someone to defend yourself or your family if there is no other way. The way you are saying it, the Death Eaters could have barged into a children's hospital and raped and tortured and mutilated every patient before killing them in a particularly painful and spectacular way and nobody under any circumstances with the sole exception of Moody
would have been given any authority to do anything to stop them except for cast a dancing jinx on them.