For the millionth time, the word "incident" in that "Sabo incident" statement was a choice made by the translator, since he didn't know if Sabo was a thing, a place, an event, a person, or anything else, he used this "incident" word there to make Sabo something generic.
In fact other translators (Molokidan, Cnet) even translated it as "Sabo thing", and Cnet even stated that it was completely vague:
"Ace: …Yeah... you know... if it weren't for the Sabo thing... and having a little brother like you to look out for... // ...I would never have... even... wanted to live...
[TN: I hate you, Oda. Okay, DISCLAIMER: I have no idea what "sabo no ken" refers to. "~no ken" is pretty much the vaguest structure ever, and I don't know whether "sabo" is a person's name, the name of a thing, or anything else that it could feasibly refer to (edict suggests that the word can be an abbreviation of "sabotage", or a type of shoes, and to be honest, I can't rule either of those out either).]"