I wouldn't have seen it as building up to the death of Ikkaku... If they wanted to do that, they would've incorporated the the flashbacks with the actual fight so that it would've had more impact. But dedicating one whole episode to a character who just exchanged a final blow with the opponent would seem kinda useless if they were going to kill him off. So, if you felt it was anti-clamatic, I think the show succeeded in fooling you with the added suspense.