Character Analysis
Courtyard Bully
Merlin's cousin (an illegitimate son of Merlin's grandpa) has no love for him. Dinias spends most of his childhood looking for ways to beat up on the nerdy, unsocial Merlin. His strength and "manliness" earn him major points with the King his father:
[…] at the age of five, stolen a ride on one of his father's horses, a wild brown colt that had bolted with him through the town. […] His father had thrashed him with his own hands, and afterwards given him a dagger with a gilded hilt. Dinias claimed the title of Prince—at any rate among the rest of the children—from then on, and treated his fellow bastard, myself, with the utmost contempt. (I.2.55)
Dinias is definitely lacking in the empathy department—and he's not the brightest bulb in the pack, either. But when Merlin returns to Maridunum after his five-year absence in Brittany, he finds plenty to pity in Dinias' condition: "About his whole person was that indefinable air of seediness which comes from relentless calculation from day to day or perhaps even from meal to meal" (III.4.44).
And even though Dinias can't hold his drink and thereby creates the conditions for Merlin to get kidnapped, there's a lot of tragedy behind Dinias' life. "He killed the children, did you know?" Dinias tells Merlin, revealing what happened to all the other kiddos who used to live with them at the palace at the hands of Vortigern (IV.2.59). It's his way of asking for forgiveness for all of his nasty behavior.
Coming Full Circle
Dinias regrets getting Merlin into such a tight spot with Vortigern. He regrets taking the money that Blackbeard gives him for unknowingly betraying his cousin: "I don't sell my kinsmen, you know" (IV.2.32).
Merlin doesn't feel like he needs to judge the guy who used to beat the snot out of him when they both lived in the same palace. He really does forgive Dinias because, after all, they are kin. And Merlin doesn't have much of that left in this particular neck of the woods.
Dinias eventually redeems himself by promising to join Ambrosius and fight for the right side for once in his life. Merlin reports that Dinias does him proud: "He kept his word, and joined Ambrosius later, at York, with a few hundred men. He was honorably received and acquitted himself well, but soon afterwards, in some minor engagement, received wounds of which he later died" (IV.2.67).
Sometimes, redemption is all a character can hope for, dead or alive.
Dinias' Timeline