necessary evil

• n. 不好但却必不可少的东西

necessary evil的用法和样例：

例句

1. A critic is a necessary evil, and criticism is an evil necessity.
   批评家是必然的邪恶，而批评是邪恶的必然。
2. The loss of jobs is regarded by some as a necessary evil in the fight against inflation.
   有些人认为要遏止通货膨胀就难免有人得失业。