1. necessary evil - Noun
2. necessary evil - Phrase
necessary evil (plural necessary evils)
(idiomatic) An unfavorable thing that must be done or accepted, especially because the available alternative courses of action or inaction would be worse.
Another punishing round of chemo is a necessary evil if Lisa is to have a fighting chance against the cancer.
My boss is a real piece of work, but my kids make keeping this job a necessary evil.
Garry Wills, A Necessary Evil: A History of American Distrust of Government, Simon and Schuster, 1999, page 252. ("Until recently, the Second Amendment was a little-visited area of the Constitution. Source: Internet