Killing itself is not 'wrong'. Some wars are justified and some aren't. War is just murder undisguised, but nations and states give the authority to individuals and send them forth to do the job. A person attcks you and you kill that person in the defense of your own life, basically, tough shit. Society has the authority under law to enforce that proxy decision. The problem with capital punishment is having the 100% assurance that we--the society--are killing the right person. There's the rub.