You really should not fold KK 6max or even full game, unless you have extraordinary reads. Your simply giving up too much to fold KK. AA vs. KK is one of those unlucky things that happens. I am pretty sure that you will lose more folding the best hand than you lose in those rare times you are up against AA. Put the money in the center and hope he doesnt have AA. Tournaments are a way different case, and folding KK is sometimes the correct play.