why is it illegal to blunder your king

I'm wondering why the rules prohibit you from hanging yor king, it would be the ultimate blunder and lose instantly. Why is this ultimate blunder an illegal move? 

Probably because checkmate ends the game. If a player could make a move exposing the king to check there would be cases where one player could checkmate first.

That and some people would try and be tricky and hope their  opponent doesn't see it. It already sometimes happens in games where it is illegal. 

The official rule in blitz is that illegal moves end the game if caught and claimed. The other player shouldn't take the king but just claim the illegal move and the win.

Read and understand the rules. It's against the rules to make an illegal move. Putting your king under attack or leaving it under attack is an illegal move, thus it isn't allowed.

I think it's based on the ancient idea that the king should be captured, not killed.  When Benjamin Franklin was U.S. Ambassador to France, he defeated a noble woman in chess after she hung her king and he took it.  She protested that you cannot kill the king.  Franklin replied that you can do so in America.

That's like a football team claiming victory because the other was offside.

Hikaru Nakamura did this once to a strong 2700 GM. He promoted a pawn to a knight, got 2 knights and although the opponent had a queen, nakamura gave some checks, and in severe time pressure the GM accidentally moved his king in check (knights are tricky!) and Hikaru instantly stopped the clocks and was given the win.

This was in a blitz OTB game.

I guess the logic is that in blitz, you just can't afford any nonsense from your opponent, because time is so sparse.

Well, it works just like a draw claim. The TD/arbiter might have to get involved if the opponent thinks that is incorrect. And of course, stopping the clock is important.

Here is the blitz game (it has an increment) where Boris Savchenko blundered his king, losing the game immediately.

Rapid has rules for illegal moves but which are in effect are determined by the number of players vs arbiters. If there are enough arbiters then competition rules apply:

1. If during a game it is found that an illegal move has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined, the game shall continue from the last identifiable position prior to the irregularity. Articles 4.3 and 4.7 apply to the move replacing the illegal move. The game shall then continue from this reinstated position.
   If the player has moved a pawn to the furthest distant rank, pressed the clock, but not replaced the pawn with a new piece, the move is illegal. The pawn shall be replaced by a queen of the same colour as the pawn.
2. After the action taken under Article 7.5.a, for the first completed illegal move by a player the arbiter shall give two minutes extra time to his opponent; for the second completed illegal move by the same player the arbiter shall declare the game lost by this player. However, the game is drawn if the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves.

If there aren't then the following applies:

1. An illegal move is completed once the player has pressed his clock. If the arbiter observes this he shall declare the game lost by the player, provided the opponent has not made his next move. If the arbiter does not intervene, the opponent is entitled to claim a win, provided the opponent has not made his next move. However, the game is drawn if the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves. If the opponent does not claim and the arbiter does not intervene, the illegal move shall stand and the game shall continue. Once the opponent has made his next move, an illegal move cannot be corrected unless this is agreed by the players without intervention of the arbiter.

Here is the game.

EDIT: Oops, I just saw you already posted the game. Sorry.

Because the goal of a war is to turn the king into a hostage so he commands his men to drop their weapons. Actually assassinating a king is unwise as the war would go on.

i think it's weird that a player won't lose the game instantly if he places his king in check in a game that is not rapid or blitz. making it legal to hang your king would also remove the need for the worst rule in chess: stalemate.

Actually, I have seen some blitz tournaments where the rule about the King is modified so that if you put your King into check, you lose automatically.