Answer
Normally they were held for ransom, check out the history of Richard the Lion-Heart for one.
There are many examples of Kings being held, rarely executed, don't forget that a King/Queen in the past were seen as chosen by God, and to superstitious people they wouldn't risk God's wrath by executing such a person.
Knockout Kings happened in 1998.
Kings of the Beach happened in 1988.
Speed Kings happened in 2003.
Devil Kings happened in 2005.
Hustle Kings happened in 2009.
Warrior Kings happened in 2002.
Battle of the Ten Kings happened in -1700.
Crusader Kings II happened in 2012.
Battle of Kings Norton happened in 1642.
Battle of Kings Mountain happened on 1780-10-07.
Bandit Kings of Ancient China happened in 1996.
the battle of kings mountain