As the article said somewhere, Iran has much bigger fish to fry that a limited Iraq person, who can be replaced and remove the threat he had also become to Iran by fomenting (or planning to foment) anti-Iran and anti-Shia disinformation, while taking Shia students and the like off a bus and slaughtering them.
Thus getting lots of Iraqi Shia protest as to what in hell Iran was going to do about it and also sickening Iraqi Sunnis, who were not as happy at the murder of their own countrymen - regardless of creed - by someone who was not an Iraqi to start with. And so staunchly supported by Iran. Gave Iran an increasingly bad reputation among those they wanted to recruit against the US and coalition.
Lots of swirling, roiling undercurrents here. Zarghawi had outlived his usefuelness. Iran operates separately from Al Qaeda and on occasion have conflicting purposes. Specially in Iraq where Iran floated a "let's work together" balloon toward the USA.
Take a close look at the various cross currents before sloughing off factual reporting.
Are you able to speak or read Arabic?