The Worlds of Katherine Kurtz

Anyone aside me ever wonder why Miklos was so eager to kill Sudrey?  I should think that if she was such a great loss to Torenth, they'd have been more eager to get her out of her captivity before she ever married into the family of her captors.

I would say it was because she had already "betrayed" her kin by marrying into a noble family of Gwynedd.

Her Torenthi kin don't seem to have cared about her or her brother for most of their lives, not to support them when their uncle died or when they were beseiged and captured.  Torenth may have known they were being held in comfortable captivity, but remember how even Javan didn't know what had happened to them after that.  Sighere's family seems to have kept her marriage to Hrorik rather quiet until Javan's coronation.
As for what made Miklos kill her in 928: wasn't it earlier that same year she'd finally agreed to a meeting with him, where she reiterrated her loyalty to Gwynedd and the Haldanes, which led to Hrorik's death at Cullicairn?  I'd agree that her "betrayal" of her race was a large part of Miklos wanting her dead, but I think her refusal to support Marek was a big factor too.

In my opinion, Sudrey's marriage to Hrorik was the main reason she renounced any loyalty to Torenth.  She never expected that she would fall in love with her jailor, but she did.  Her refusal to help Miklos in any manner, I believe, is why he decided to kill her.

Another point is that Sudrey is a potential rival.

The Princes of Kheldour are a branch of the Festillic line and the line of succession after Marek and his son are the Furstan-Festil-Fathane lineage - not particularly prolific at this point, at this point possibly Count Kalman and his son Istvan - and then Sudrey and her daughter, descended of Festil I's younger brother.

Given Marek's tenuous political position (King Arion doesn't appear to have been fond of him) and dubious legitimacy - he is after all, a bastard - he might well want to show he can 'keep his house in order'. Otherwise if he mis-steps Arion can simply get rid of him and either put up the Count of Fathane as the new, legitimate heir to Gwynedd or simply pressure the Count to step aside in favour of the Howells - rendering it an internal Gwynedd matter.

Neither's particularly likely but Marek has every grounds for paranoia.

Or maybe he was genuinely furious at not getting support from one of his closest remaining kin.