Thomas Aquinas Commentary Lamentations 4:13

"[It is] because of the sins of her prophets, [and] the iniquities of her priests, That have shed the blood of the just in the midst of her." — Lamentations 4:13 (ASV)

Here he gives the reason for the destruction, so that the astonishment would cease.

And first, there is the fault of the prelates: of her prophets and priests, who should have governed the people. Iniquity came out from Babylon from the ancient judges, that seemed to govern the people (Daniel 13:5).