difficult to answer eh? true for us but not for God,two scriptures come to mind at first thought,the commandment,"thou shalt not kill" and the scripture"vengeance is mine sayeth the lord" so both of these bits from the bible tell that killing is wrong for us, because only God is justified in slaying anyone, because if you think about it,we are his children so it's only fitting that he be the one to decide upon if anyone of us is worthy of death because it is a hard thing for a father to lose even one child to evil yet God as our father is still long suffering for us all to be led to repentance and be saved,if you doubt this read of the destruction of sodom and gomorrah,Abraham be sought God to not destroy it for the sake of the righteous particularly Lot,pleading that if there were a certain number of righteous people in the city that God would not destroy it,Abraham pleaded til God agreed at the least that if there were a mere "ten" people in the city that were righteous that he would not destroy it,yet not even that small number of righteous people could be found,but the main point is God Loves Us and is not as hasty as we are to think on killing yet when it comes to this point of unavoidance he will act upon it swiftly and justly when there is a need,upon the wicked.