Nice display of sportsmanship. I'm happy to hear everyone kept their word.
Fair play
The capturing king rule is not a bad rule. There wasn't really any reason to turn it into a weird 3 player chess game.
I think it is admirable to do this, but I don't think this should become the norm or an unspoken rule. If someone has a really good opportunity to take the king and does not want to agree with this, then I don't want that person to be considered a jerk. If someone say they won't capture the king and does it anyway, then sure that is not nice, but I would not blame someone for something they did not agree to.
Just played a game with CollisionofMinds and Oily_GOD. The 4th player timed out at the very beginning of the game and we decided to leave his king untouched. So, instead of fighting for those +20, we played an interesting and fair game. Special thanks to CollisionofMinds who offered this.