Apple, if I recall correctly, after Woltz calmed down, he saw not only the ruthlessness of killing Khartoum, but the calculation behind it--what was the penalty for killing a horse in California? He also thought that the Khartoum decapitation showed that the Corleones wouldn't kill him: "they had something more clever and painful in reserve." I took that to mean that Woltz, the old pervert and pedophile, was afraid that the next item to be severed by the Don's henchmen would be his gonads. If so, that'd be an even more subtle message from Vito: No more screwing that underage starlet, Woltz! Or anyone else!