Why "double-cross” means to betray ?

Question

Why 　"double-cross” means to betray ?

Answer 1

You make the sign of the Cross to swear your word to some oath, then behind your enemy's back you make the sign of the Cross and make another vow that voids out the first "crossing".

Answer 2

The term double cross refers most usually to a betrayal. To double cross someone means to suggest that you are an ally when you are not. The phrase originates from the use of the word cross in the sense of foul play - deliberate collusion to lose a contest of some kind. Originally, the phrase was used to refer to either of two possible situations:

1. A competitor participating in the fix who has agreed to throw their game instead competes as usual, against the original intention of their collaborators - one "cross" against another.
2. Two opposing parties are approached, urging them to throw the game and back the other. Both parties lose out, and the perpetrators benefit by backing a third, winning party.

This use has passed into common parlance, so that, for example, in World War II, British Military Intelligence used the Double Cross System to release captured Nazis back to Germany bearing false information.

Answer 3

double as in the betrayer betraying the other and cross is for going from one side to the other which is betrayal