Certain expressions join words which should immediately cause us to pause and think long and hard. In my mind "necessary evil" is a powerful example. Admittedly, sometimes actions normally viewed as evil might be our only available option in a particular situation. Thus we would have no "good" choice. Such situations define one type of what we might call "necessary evil", a situation which requires action but for which there is no possibility of action which does not harm someone.
A specific example of that described above is the classic hypothetical scenario of the runaway train being observed by an individual standing next to a switch directing the train to the right or the left. To the right the train will kill family members. To the left the train will kill a much greater number of strangers. What must the individual at the switch do ethically? The only two available options are to do nothing or throw the switch. Either way something very bad will happen, without a doubt.
However, this situation is different from another typical type in which normally unacceptable behavior is justified as a "necessary evil". This alternative scenario is active whenever we decide to react aggressively and forcefully to a perceived threat or offense, to eliminate the threat and/or to seek retribution for the offense. In such cases there are always more than two available options: do nothing, grant forgiveness, seek reconciliation, or respond violently out of a sense of moral outrage.
The moral conundrums illustrated in the first case arise occasionally in reality but exist more as hypothetical examples. Thus most of us never deal with them.
The moral/ethical situations described in the second case arise every day in the lives of almost everyone. Often the definition of a necessary evil is not even applied to these events. Instead we justify a violent response as an exercise of righteous indignation, self preservation, or justice. Verbal abuse, physical beatings, even killing of a human being are routinely given at least our mental approval on these grounds. In fact, those who respond to perceived threats and offenses violently, actually see themselves as righteous and morally superior in doing so. By implication any who fail to respond the same way to the same perception are unrighteous. For someone to fail to even perceive or acknowledge the same threat or offense is viewed as even worse.
Any time we behave contrary to what we would universally view as proper- politely, reverently, patiently, graciously, kindly, humbly- any attempt to justify that behavior will involve an implied necessary evil, something normally avoided because it is sinful but which in this case is deemed essential to survival or a sense of justice. Justifying preemptive and retributive violence with labels like righteous indignation and justice has a long history. In so doing we redefine righteousness in order to exonerate ourselves in acting counter to normally accepted norms. We, thus, become a righteousness unto ourselves, the epitome of self-righteousness.
Whatever moral and ethical situation may arise, we will rarely, if ever, encounter one in which our choices are limited to only evil options. Conversely situations calling for either retribution or forgiveness arise regularly. We can take the route that proposes a necessary evil, but just be aware that the label itself will never make your actions righteous. Also when evil becomes a necessity in your mind, you have become your own Satan.
Our traditional religion is the greatest purveyor of the idea that necessary evil is actually righteous. This is so because the church sees itself as the suppressor of the unrighteous. The methods of suppression, no matter the nature, have routinely received church approval The religious institution which claims to oppose evil does so with no hesitancy to promote "evil" in that opposition. Conflict after conflict has been initiated and/or encouraged by the church. In effect, the church has made itself a necessary evil, something to be endured rather than celebrated.