It must be your medication Roxy. I checked it out right now. It's as clear as day - in hindsight - watching the scene at the end of D&D what Derek was thinking. I'm bemused that you can't see it.

Derek: `...We can fix all the mistakes.'
Soldier: `How are we gonna do that? Reece, how are we gonna do that?'
Reece looks across at Billy Wisher.
...
The very last scene in D&D is Derek assassinating Andy.

You are paddling way deep in that well-known river in Egypt if you put another interpretation on it.