Why does everything that happens in Isaiah—or, all the wrathful stuff, anyway—need to happen? "Sin" is one answer. God needs to punish everyone's sins, hence the Assyrians invade and get slaughtered by an angel, hence Babylon falls and the invaders massacre civilians… and so on and so forth.
But what is sin? It takes all kinds of forms throughout the course of the book, but it pretty much always involves forgetting God, forgetting the origin and the creator of humankind. The Assyrians help God punish Israel for turning away, worshipping other gods, and failing to behave justly (to take care of widows and orphans, in particular). The Assyrian King Sennacherib is himself punished and destroyed, since he openly believes he's superior to God himself. So sin, at its most basic sense, involves turning away from what's really central and important. It comes from getting involved in lots of distractions and egotism.
Questions About Sin
What are the worst sins anyone commits in Isaiah? Are there any sins that aren't so bad?
Are there differences between the way Judah and Israel sin (or, the kind of sins they commit) and the sins that the other nations commit, like Assyria and Babylon?
Why does pain help purify sin? Could there be a more pleasant way of making things right?
Could people simply avoid the bad consequences of their sins by turning and be good? Or does there need to be a reckoning?
Why do people in Judah (or in Assyria, or wherever) keep sinning? What do they get out of it?