Was slavery thought of as a moral issue or an economic reality in A Necessary Evil?
A Necessary Evil was written by Garry Wills, a professor and popular author, in response to the 1994 congressional election. Although he himself is Roman Catholic, and early in his career wrote for many conservative magazines, he saw the Republican Party's "Contract With America", written by Newt Gingrich and Richard Armey, as a triumph of the right wing anti-government faction in the Republican Party, a faction he considered as grounding its policies on a mistaken interpretation of United States history.
Slavery is not a major focus of his book, as his primary concern is arguing that government, rather than being a "necessary evil" is a "necessary good." Although his main focus is on the Revolutionary War, the Constitution, and other early developments, he does address the Civil War mainly in terms of how secessionists in the Civil War contributed to the evolution of the anti-government ideology that still dominates much of Southern politics. In this way, he is really thinking about slavery more in terms of how it functioned as the starting point for debates over states' rights than in either moral or economic terms.
In general, though, slavery had been a part of so many societies for so many thousands of years of recorded history, that I suspect people in many parts of the world in the early modern period simply accepted it as a common cultural tradition. However, as the abolition movement grew, it brought into focus that there was a moral component, and as people began to think about that, many began to consider slavery morally wrong.
The theories of Charles Darwin, which emphasized the common ancestry of all people, contributed in many ways to the revulsion against slavery.
Are you asking if slavery was "a necessary evil," either as a moral issue or as an economic reality?
"Necessary" would be difficult to defend, I would say. But slavery did support the southern economy, as it was structured at the time, allowing many families to keep farming or to farm more land than they would otherwise have been able to. "Beneficial-to-certain-parties," would be more accurate, certainly, than "necessary."
As a moral issue, there were absolutely no redeeming qualities of slavery, so I am assuming you're asking if resistance to or rejection of slavery was "a necessary evil." And, to that, I would of course give a resounding yes. Change does not happen in a vacuum, and the status quo is always easier than any new system, so fighting against slavery was not only morally the right thing to do, but it was also the only way to cause positive changes to the societal structure of the times.