Angels have Free Will?

 Ohr Hachayim (Bereishis 19:20) We also have a principle (Baba Kama 60) that once permission has been granted to the destructive angel to execute G'd's judgment, it does not really matter whether the measure of guilt is full. However, the angel Gabriel was then in a position to exercise some discretion. Lot was aware of this and appealed to Gabriel's discretionary powers. Lot left Tzoar as soon as possible because he was afraid that the time limit for the angel's discretionary power would elapse.

Radak (Bereishis 19:21) When God sends an angel He gives them permission to add or subtract slightly from their mission as they understand it  because they are intelligent beings

 Rashi (Bereishis 19:22) FOR I CANNOT DO ANY THING—This admission of their powerlessness was the angels’ punishment for having said, “For we will destroy the city”, attributing the act to themselves; therefore the incident could not close until they were compelled to admit that the thing was not in their power