>>512952034
Historically, holding royals hostage has been a generally common tactic. Their value, after all, comes from the fact that they are alive. There is rarely a time pressure to rescue captive royals because its not like the hostage-takers would go anywhere with them either, only one country cares about the state of its own royalty and other nations will only want them so that they can return them for leverage/favors/ransom.
The idea of just slaughtering the entire family was, at the time, preposterous. A mob of furious peasants falling upon the king as a scapegoat for their troubles is one thing, but an actual organized army with all the cards in their hand choosing to discard the royal line for no benefit?