If at any point during the investigation I thought the killer was going to harm the informant or anyone else, I would have arrested him immediately and taken my lumps in court for having brought a less-developed case. Fortunately, I was able to use the next six months to build an airtight case against the killer.
It seems reasonable to assume that the agents in the Zazi case acted out of concern for public safety, possibly because they had a foreign intelligence wiretap on Zazi's phone. If they had information that Zazi was going to harm citizens - whether from an informant or a wiretap - they had to make an immediate arrest.
Unlike my murder case, they may not have had the luxury of time to finish their investigation before they took Zazi into custody. If the evidence presented against Zazi in court turns out to be less strong than it might have been with the luxury of time, but the agents averted another 9/11 when they arrested him, any criticism of them at a later time will ring hollow indeed.
The law enforcement and the intelligence communities have been dramatically overhauled since 9/11. Whether these improvements resulted in uncovering Zazi's alleged terror plot is unknown. The 9/11 Commission report noted that the FBI had a limited ability to collect and analyze intelligence, and share evidence with intelligence gathering agencies. The FBI might have disrupted the 9/11 attacks had they connected the movements of Mohammed Atta and the other hijackers through intelligence sharing.
The FBI of today does collect and share intelligence. Perhaps the new FBI could have discovered that three of the four Hamburg, Germany cell members had arrived in the U.S. during the summer of 2000 and begun pilot training.
Apparently the Zazi plot was uncovered because of these improvements. If so, this will be a welcome change.