It's difficult to appreciate the ebbs and flows of a major event like the Iraq War while it is still ongoing, and the battle against the insurgency there is undeniably still a brutal, daily fight. Nonetheless, historians some years hence will likely identify that the beginning of the end of the insurgency occurred well before today and that al-Zarqawi's killing was essentially a highly visible event during an inexorable decline for the insurgents and their supporters.
al-Zarqawi's death is a major step toward the end of the insurgency, but not because al-Zarqawi remained the key figure in that fruitless effort; many sources have reported that his control over the insurgency and its supporters was waning and that he had become a marginalized figure outside his faction. It's certainly fortuitous that his death occurred at a time when the will of a substantial portion of the American public to continue in Iraq was getting a bit shaky, but timing alone doesn't make this event important.
Ultimately, al-Zarqawi's killing will herald the end of the insurgency not because of al-Zarqawi's importance, but because his death resulted from cooperation between the Allies and moderate elements in Iraq and the broader region who were alienated by the bloody tactics of the insurgent forces. According to Time, Jordan played a crucial role in providing necessary intelligence to the operation:
In the end, the savagery of Musab al-Zarqawi may have earned him too many enemies.
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A well-placed intelligence source in Jordan told TIME that the CIA was tipped off after Jordanian intelligence learned of a meeting that Zarqawi planned to hold in the town of Baquba, north of Baghdad. His safe house was targeted in an air attack, and, says the same source, the Jordanian-born leader of al-Qaeda in Iraq was killed in the bombing. A senior Jordanian official confirmed that "there was a Jordanian security role in this."
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[S]ome of the more nationalist leaders of the insurgency who had been quietly negotiating with the U.S. and Iraqi government had made no secret of their animosity toward Zarqawi and the al-Qaeda agenda. The announcement, just a day before Zarqawi's death, that the new Iraqi government would release some 2,500 Sunnis imprisoned for assisting the insurgency, suggests that rapprochement between the government and the Sunni nationalist element of the insurgency may be accelerating, which was bad news for Zarqawi.
"Bad news", indeed.