They believed that it was a necessary evil.

VOA: special.2011.04.07

Some would get worried and they would discuss slavery as a necessary evil this system entailed upon them.

有些人感到忧虑,他们认为传承下来的奴隶制,毫无疑问是种罪恶

耶鲁公开课 - 美国内战与重建课程节选