It seems that the primary reason behind Ace's execution was him being Gol D Roger's son. Roger's infamous reputation and legacy must be destroyed or so claims the navy. Being the late Pirate King's son Ace was such a huge threat that even level six of Impeldown was not enough for him. Now my question is, Why Shiki, who was once a rival of Roger (reputationwise) was not excuted? I mean his threat level was neary as big as Rogers. So was it not normal for the world goverment to execute him too? But they dd not, why?