There are two faces to "necessary evil." The obverse is something like spanking a child who has unknowingly risked its life in some dangerous activity. It is very necessary, and it is devoid of true evil, in fact, it may in future save the life of the child.
The reverse, however, is called "necessary", but its true emphasis is evil. Such as Stalin's quote that "You can't make an omelet without breaking eggs." (Noting that he broke a vast number of 'eggs', yet never made anything like an omelet.)
So the first case is not evil, and the second case is not necessary.
Even in an extreme, like war or capital punishment, the term "necessary evil" may be an oxymoron. I could cite the truly revolting cliques of past, such as the Thuggee and the Aztecs, where the only good that could arise from them was by their annihilation.
Or I could even use those leaders of Iran who both crave nuclear weapons and so much as state they intend to use them. They should face destruction alone, but they hope to drag their people with them.
But even in that, their people are not without sin, because the man on the street in Persia believes that nuclear weapons will be like Aladdin's djinn, and that all their wishes will come true when Iran has such weapons.
So there is no evil involved in stopping such a thing. And even greater good if it involves minimizing the number of innocents whose blood is shed in the process.
It is not evil, but beyond any doubt, it truly may become necessary.