For all he knew, he was just sentencing an uppity defenseless and talentless kid to death, and had the assassination attempt succeeded, she would never even reach his territory, meaning there's no reason for him to even think his family members could be harmed. And Mythia did say she's going to 'return the favor' and that probably made him think she would actually do terrible things to her granddaughter which caused him to panic even more.
My primary point is that he's being painted as this ruthless criminal mastermind eugenicist who looks down on anyone he thinks is beneath him, and that he's a heartless genius seeking power and control.
If he though Mythia was "an uppity defenseless and talentless kid", then he's not nearly as intelligent as he thinks he is, to not spend
any time looking into her to gauge her actual abilities and potential.
That, and he really shouldn't have any weaknesses that could be exploited, if he wants to stay in his position. And that includes things like the safety of family members. The fact that he could be visibly shaken by the fact his granddaughter was captured, and even at the threat of potential harm coming her way, says he lacks the conviction to carry out his schemes, if that can sway him.
It's understandable that it could for most people, but he's self-styling as some omnipotent bastard who'll scheme and crush anyone in his way. Him suddenly freaking out because another person is captured diminishes that greatly.
It
humanizes him, but all I'm saying is that it also just makes him a coward and greatly undercuts whatever prestige he was building for himself, and makes him pathetic. A
real bastard antagonist wouldn't let the death of a family member dissuade them from caving on their goals/ideals.