There is no world in which Tom Cotton saying that slavery should be taught as “the necessary evil upon which the union was built” is being taken out of context. Here’s the context.
In the interview, Cotton said the role of slavery can’t be overlooked.
“We have to study the history of slavery and its role and impact on the development of our country because otherwise we can’t understand our country. As the Founding Fathers said, it was the necessary evil upon which the union was built, but the union was built in a way, as Lincoln said, to put slavery on the course to its ultimate extinction,” he said.
Instead of portraying America as “an irredeemably corrupt, rotten and racist country,” the nation should be viewed “as an imperfect and flawed land, but the greatest and noblest country in the history of mankind,” Cotton said.
Notice that he doesn’t say, “We need to study how the Founding Fathers said it was a necessary evil.” He says, “As the Founding Fathers said…”—a grammatical construction one uses when one is citing a source to support one’s own view.