Musar על בבא קמא 119:17

If we want to answer this in a simple way, all we have to know is that there is no remedy against G–d's own decree. Once G–d personally has decreed that so and so is going to die, no incense or other remedy can prevail against such a decree. The secret that the angel of death revealed to Moses, concerned matters not decreed by G–d directly, but which the angel of death saw fit to include in his general authority to kill, without regard to guilt and innocence, but within the area of his authority. In the plague described here there were many people whose death had been decreed by G–d personally. The angel of death had come to kill these people. Aaron tried to stop him, but to no avail. This is why we are told how the angel of death responded to Aaron's attempts to restrain him by telling Aaron about individuals whom he was empowered to kill. His argument was that he killed these people at the specific instruction of His Employer. He told him that no incense can prevail in situations such as these. He claimed that the secret he had revealed to Moses was effective only when he was not killing at the specific command of G–d, but only within his own general authority. Aaron countered that the authority of Moses was such that he had succeeded on previous occasions to reverse fatal decrees that had been issued by G–d directly and specifically, and that most likely he could also do so in this case. He therefore pleaded with the angel of death to wait. This would be an appropriate explanation, except for the fact that in our situation the very sin had involved use of, or rather misuse of, incense. Therefore incense was not thought to be effective here to ward off the angel of death. The Israelites then complained to Moses that he had neutralized the very secret that the angel of death had revealed to him by advising the two hundred and fifty men to prove their case by offering incense.