A way to fix it? (SPOILERS)
Don't get me wrong, I don't think Righteous Kill was a bad movie, I just don't think it was a great movie. With the actors involved, it should have been. And, yes, I mean the long-awaited Pacino-DeNiro second coming, but the supporting cast are great, too.
But I think what's fundamentally wrong with Righteous Kill is that the movie is all about the plot twist. It's a Whodunit? where the main reason to watch is to get the answer to that question. The best Whodunits? are fun to watch in the journey, not just for the reveal.
But with Righteous Kill, I say go with Hitchcock's maxim of building tension: tell us.
If the movie showed us, right from the start, who was the killer and then let the rest of the movie be about us knowing that this mega-secret could be uncovered by the other cops at any time, it would go from being a mediocre Whodunit? into a brilliant thriller. Every scene would have been reconstructed to build that tension. We'd know that DeNiro and Pacino are in a duel in every scene. I think this would have forced the filmmakers to stop writing "Oh, who is it!?" scenes and start writing scenes where we sit on the edges of our seats waiting for the big revelation to come out.
More than that, they'd be able to write scenes where we could re-watch the movie and think, "Hey, does he know his best friend is the bad guy yet? He could know right now and just be in denial." They could have given us scenes that we could endlessly repeat.
Why were we looking forward to this movie so much? Heat already took the "First ever DeNiro/Pacino movie!" hype. So, what was left for Righteous Kill? More of that pairing. But what made Heat's one scene so memorable was the interplay between these two titans of acting. The scene wasn't written to set up some goofy plot twist. The diner scene was set up to watch the interplay of people who loved and hated each other. Best frenemies meet face-to-face. If Righteous Kill stopped trying to fake us out, it could have been a whole movie of that dramatic tension.