User blog comment:Blackendedsoul/Why not Shiki?/@comment-220.255.1.56-20110322213218/@comment-4605926-20120620191217

Here's my idea concerning this dilemma, as the Pirate King aka Roger, is known worldwide more than Shiki (at least to the simple civilian), a part of the execution's idea is to get the public to see that the really biggest threat is no more and (my opinion) to get the public's attention and to ensure everyone thinks well about the Government/Marines for future purposes. When the Marines realized Ace was Roger's son, they received this second chance. So yea, I guess it's more for the public than for the general safety in the world. Roger's still just 1 guy and he can only be in one place at a given time so getting rid of 1 high threat criminal is less than getting rid of groups with lower threats. Don't forget that one of the World Government's top priority goals is its general status or its reputation in the eyes of the public which is also one of the reasons the war at Marineford was broadcasted (even though they didn't initially know about their victory) and the reason why they wanted to keep Whitebeard's and Ace's body which, as the Marines claimed, were a symbol of their great victory.

Just one thing I'm not too sure of for the end, don't they execute every criminal they manage to get their hands on and just not make too much of a fuss about it? I'm not too sure where I got that from so I just wanna confirm it.