How fast would that kill?
Ingestion of radioactive material would not kill that fast, only massive exposure to very high levels of gamma would. At those levels, the airport would be a graveyard.
How fast would that kill?
Probably not very fast. A dose of radiation high enough to kill in minutes would be impossible for anyone to handle without shielding. A lower (but still lethal) dose could be handled for a period of time (depending on the dose), but would also take longer to kill. Plus, in an airport, they probably have radiation detectors.
On the other hand, there are plenty of ways to deliver a lethal dose of a chemical poison without killing the person delivering the poison.