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.

Question: Why was that an appropriate description?
Self-contained rewrite: Why was the description of Maelgwn as a dragon an appropriate description?