Conversation so far:
What was Gildas?
Gildas makes an allegorical condemnation of 5 British kings.
Why did he condemn them?
likening them to the beasts of the Book of Revelation.
What did he say about Gwynedd?
describing Maelgwn as the "dragon of the island" is appropriate.
Why was that an appropriate description?
The Isle of Anglesey was the base of power of the kings of Gwynedd.
Why did he make these condemnations?
The reason for Gildas' disaffection for these individuals is unknown.
Was there any speculation why he made this comments?
suggests a reason other than his claim of moral outrage over personal depravity.
What suggests this?
That he chose only the kings associated with one king's pre-eminence.

Question: Which king's pre-eminence?
Self-contained rewrite: Which British king's pre-eminence?