Conversation so far:
Who was Gildas?
Gildas was a 6th-century British monk.
Why did Gildas condemn his contemporaries?
Gildas' contemporaries, both secular and religious, are condemned because he blames them for the dire state of affairs in sub-Roman Britain.
What did Gildas say about Maelgwn Gwynedd?
Gildas says that Maelgwn Gwynedd is the dragon of the island and goes on with a litany of moral accusations.
Why was the description of Maelgwn as a dragon an appropriate description?
The Isle of Anglesey was the base of power of the kings of Maelgwn Gwynedd, so describing Maelgwn as the 'dragon of the island' is appropriate.
Why did Gildas make these condemnations against five kings?
The reason for Gildas's disaffection for these individuals is unknown.

Question: Was there any speculation why he made this comments?
Self-contained rewrite: Was there any speculation why Gildas made these comments against the five kings?