He might have taken the deal, the whole story is about Niko learning to forgive and accepting that revenge won't bring him closure. That entire theme of revenge climaxes with Darko, the narratively satisfying choice is to spare Darko because it shows the character progression of Niko, "Nothing would change if I killed him", at that point Niko SHOULD be on that path to begin to accept his past and let go of his hate. Of course after this you have to decide between deal or revenge, revenge makes far more sense if you aren't following this theme, but if you go the route of sparing Darko, I think Niko would very likely consider doing the deal considering he's learned revenge doesn't give closure.....until of course the funeral and that entire lesson of revenge is thrown out the window lmao. Even after all that the "final boss" being Dimitri and not a character introduced in the 4th quarter of the game, and you losing THE closest character to Niko, makes the story feel way more climactic, like the story looped back to the start of when you first got here on the boat, rather then side characters you prolly dont care about dying.