A pair of kings is a very strong hand preflop. With a pocket pair KK to play better against one or two opponents. So on the flop a pair KK must be played aggressively to weed out players. From any position, you need to raise to 3-4 BB. If the flop comes an ace, then you should check the strength of the opponent's hand by raising. If the answer is a reraise, then it's better to fold.