Brann of the Rangers is initially presented as a bad guy and even answers exactly the same physical description as Varagan except for eye color. See here. However, Anderson shows us good sides of Brann - as he usually does with his heroes' adversaries.
Fictional villains do not often see the error of their ways. Father Brown converts Flambeau but this is unique - I think. Aycharaych, if he survived, would no longer have had any reason to continue working for the Merseians but that is not the same thing. We never see him regretting all the suffering he has caused.
Jerry Pournelle's and SM Stirling's Geoffrey Niles does start to see the error of his ways:
"It had seemed so romantic, help the poor against the Spartan aristocracy, overthrow the tyrants, but the Spartan kings weren't tyrants. Not at all.
"He thought of what Skilly had ordered. Kill all the prisoners. His troops would have obeyed, but of course he hadn't transmitted that order.
"I was on the wrong side. This isn't Lawrence of Arabia. No romance here. This isn't anything I want to be part of."
-Jerry Pournelle and SM Stirling, The Prince (New York, 2002), p. 847.
It is good to see a character who has got things wrong starting to get them right. Does anything like this happen in Anderson's many works?
Lastly, the Smallville TV series presents two very interesting character transformations:
Clark Kent's dishonesty gradually transforms Lex Luthor from his friend into his adversary;
meanwhile, Lex's father, Lionel, somehow changes from a really bad guy into a really good guy, prepared to keep Clark's secret and even to sacrifice himself for Martha Kent.
I really wish that Poul Anderson had applied his skills as a hard sf writer to a Superman novel.