Well, it is apparent that Kizaru's appearance in the flashback wasn't too special. Judging by the note at the end of chapter 623 saying, "Kizaru in his younger days!! What are his true intentions ...!?" I thought for sure it was going to be something big. But, as it turns out, Vice Admiral Borsalino only dragged Arlong to a Marine base for interrogation and nothing more. I suppose that his purpose in the flashback is just to show us what he looked like as a Vice Admiral.
Well, Kizaru actually did a good thing by capturing Arlong. If Kizaru wasn't at Foolshout Island, many people would have been killed.