Other than kidnapping with the intent to murder the Grandmaster
Only it wasn't to extract concessions. Medb planned the scenario knowing full well that the terms for his release were beyond anything that could be accepted or negotiated away from. So fabricate a situation that allows them to murder someone and transfer blame. She admits that much on her own.
"We had to kill him because you didn't meet our demands" is right up there with "you agreed to a death duel because you weren't willing to die like the pathetic little human we need you to be."
No life debt from Medb and the father dies. She tried to sucker the MC into erasing that little bump in their plan at the release meeting.
I'm reluctant to call kidnapping the MC's father a "crime" because it's unclear just how bad relations were between the two groups at the start of the game. It's certainly an act of war, but the morality of war is complicated, and if relations had already broken down acts of war are the nature of the beast. They obviously cause massive problems, but they don't tend to spawn virulent hatred the way a "crime" (like massacring prisoners or assassinating members during peacetime) would.
Plus, if relations
were bad enough to be on the brink of war, it's entirely possible the Templars had already made their own acts of war and the Alfar were merely upping the ante. We just don't know. I have no love for the Alfar, but that doesn't automatically mean the Templars are better.
As far as their plans for the Grandmaster, I don't think the Alfar were
planning to kill him. They knew their initial ultimatum was almost certain to be rejected, but my impression is they figured they could eventually negotiate with the Templars for some lesser concessions in exchange for his release. If that didn't pan out then they would exploit the vulnerabilities that losing the Grandmaster would create when they went to war, but even then they probably wouldn't kill their prisoner since he could be a useful bargaining chip if and when they wanted a cease fire.
None of which means they wouldn't have killed the MC's father if they thought it would help them or he was likely to escape. They absolutely would have. I just don't think that was their primary goal.