I enjoyed the scene/ending in question, so I never gave much thought to how he toyed with the POV. Thinking back, I can see what you're talking about. Darrow himself plays out the lie in his own thoughts, which is a bit odd. If you're stretching, you could argue that Darrow was method acting (if that's what it's called?), and by pretending Sevro was dead and the betrayal was real, it helped him portray those emotions better.

Likely it was just Brown finding his route to be an easier way to write the betrayal without tipping the reader off.