User blog comment:LuffyPirateKing/When The Sun Goes Down/@comment-3119203-20120114215614/@comment-3119203-20120114221736

Ok, I was explaining the WG's motives, not saying they did anything good.

As for Ace, he has everything to do with it. The son of a famous criminal is likely to be a criminal, especially when that person happens to have a D in their name (the Gorosei know that D=trouble, as shown by Dragon being Garp's son). If there was any chance at all that Ace would create more chaos, then he had to be eliminated. You might say that everyone COULD cause madness, but Ace was more likely to do so, because he was the son of the Pirate King.

Even if Ace didn't cause trouble, he would still be dangerous for having his father's characteristics: strength, recklessness, and protectiveness. All of those are dangerous to the WG.