hashkafah philosophy - Free will a prerequisite for punishment?

I read that the angel Metat was subjected to 60 pulsei de-nura (Aramaic: פולסא דנורא "lashes of fire"). I have been taught that angels do not have free will. Did this angel act from free will or did he get the punishment despite that he could not have done anything about it? My real question: is free will a prerequisite for punishment?

(Chagigah 15A v12) He saw that permission was granted to Metatron to sit and write down the merits of Israel. Said he: It is taught as a tradition that on high there is no sitting and no emulation, and no back, and no weariness. Perhaps, — God forfend! — there are two divinities! [Thereupon] they led Metatron forth, and punished him with sixty fiery lashes, saying to him: Why didst thou not rise before him when thou didst see him? Permission was [then] given to him to strike out the merits of Aher. A Bath Kol went forth and said: Return, ye backsliding children — except Aher. [Thereupon] he said: Since I have been driven forth from yonder world, let me go forth and enjoy this world.

Answer

The Ohr HaChaim writes (Bereishis 1:1 note 3):

אמרו ז''ל (חגיגה טו.) במעשה מט''ט שדנוהו לפני אלישע ומחיוהו שתין פולסי דנורא, וגם אליהו וכו' (ב''מ פה:) וכמה משפטים לשרי מעלה, והגם שאין להם יצר, עם כל זה ימצא בהם הטעות, כי לפעמים לא יכוונו אל האמת וישגו, וצא ולמד ממעשה המלאך מט''ט ואליהו כי שגגו וכן יקר מקרה ויענישם ה' אפילו על השוגג מה שאין שופט במשפט זה ליושבי ארץ

Chazal said (Chagigah 15a) concerning the story of Metatron when he was judged ... with sixty fire lashes ... and many [other] judgments to the officers above [i.e. angels]: Although they don't have free will, one may find error in them, because sometimes they wouldn't understand the truth, and would sin accidentally. Come and see from the story of Metatron the angel and Eliyahu, that they sinned accidentally. And similarly, an occurence may occur where G-d would punish them even for accidental sins more harshly than what he judges in judgment for dwellers of the earth.

Basically, Hashem punishes angels for doing something wrong beshogeg (much like people are punished for things beshogeg, although it is a lighter punishment).