Sam lied betrayal was a clever way for ND to scratch the "family betrayal" itch stories like this always trope their way into, without actually having the "And I've been a villain all along...brother!!" "twist" that another storyteller probably would have gone with. The betrayal actually comes off as more sad, to me. By the time that's revealed in the game, it's at a point where we see that Drake's heart isn't as into the thrill of the adventure as it was in the beginning, but for Sam, that thirst is all he has. He doesn't want Avery's treasure because he wants it. He wants Avery's treasure because he needs it. Nate has Elena to go home to, and Sam almost put an end to that if we hadn't seen, over the course of three, now four games, how much Nate and Elena are clearly devoted to, and in love, with one another. Sam is jealous of his brother in a lot of ways, but not so jealous that he'd commit fratricide over it. That seems like an extreme reaction to a pulpy adventure series like Uncharted. Anyway, my point is that Sam is a sad character, and even in the end, he still hasn't learned his lesson. He and Sully go off to have god knows how many adventures, but I think that's fine for Sam. He's happy with that life, and he learns that Nate is happy with his. And then you have recipient of the "Best Wife of the Year" Award, Elena, supporting her husband in a way that enables them to have fun adventures, minus the gunfights, and collapsing ruins. As the character is presented, the lie betrayal feels true to the Sam we see in the flashbacks. He loves his brother dearly. (as an aside, I totally knew Sam was full of shit from the moment he said, "I'm in trouble, little brother," after showing up at Nate's office) I think having Sam be antagonistic from the outset could have worked, but was unnecessary for what Uncharted as a whole is, and for the character of Nathan Drake. What would a brother out to kill him have really added to the character or development of Nate, and how would it have really separated him from the other antagonists he'd come across in the past besides the "Oh snap, it's his brother!" dun dun dunnnn! type twist?
Overall, I think the antagonists of Uncharted 4 aren't nearly as important as Drake battling between two people he clearly loves. Sam may have been gone for 15 years, but he's still his brother. Rafe and Nadine existed solely so that we'd have a reason for third person shooter gunfights and the expected final boss fight that video games are apparently supposed to have.
