There's not much sex in Isaiah, and pretty much no drugs—unless you count illicit drink-mixing competitions—but there's a ton of violence. It's not quite as psychedelic and disturbing as the violence in Revelation, but it's definitely not toned down either.
Maybe the best example is God's prediction of what will happen to the Babylonians, since it involves the slaughter of every man, woman, and child—including infants—in Babylon. There's plenty more where that came from. Massive destruction is similarly visited on the Egyptians, Moabites, Damascus, Ephrain, Assyria, and plenty of others. In Assyria's case, 185,000 Assyrian soldiers are killed by the Angel of God in just one night.
Technically, you might be able to get away with mass battle casualties at a PG-13 level. But the intensity of the violence and the attacks on civilians puts Isaiah squarely in R-Rated territory—funny for a book that has some of the most famous descriptions of a future peace in world literature.