totally agree, but i think Veramonte worried what Jurion could get up too if kept alive. just cause u lose ur powers, doesn't mean u can't hurt the family in some way. & as Kayden said they were weaker in their current state (which who knows how much time it would take them to recover).
so i think this is for the best - even thought like u, think his punishment was to swift & in his case i wanted him to suffer more. but i think this is the better choice.

Jurion losing his powers would have been a fitting punishment for him. Having to exist as the regular human beings he repudiated would have been very satisfying. Killing his own grandson, whom he probably still holds affection for, seems more of a punishment for Veramonte