It Has To Be This Way

Remember that in the end, they both had the belief that the strong should dominate the weak, if not outright slay them, despite Raiden's denial throughout the game. If you recall, Armstrong's death sees his body in Raiden's shadow, and given that It Has To Be This Way implies Raiden and Armstrong are, in the end, one and the same when it comes to their morals, this could indeed be Raiden's fate.

I mean, could it be?

Red Sun - I think it's a lot more upbeat because Sundowner is a lot more open and honest, as well as at peace with himself, than other bosses are. That's why it's a lot more "happy" than other soundtracks. Sundowner contradicts past game villains, and Metal Gear Villains, by being bluntly open in comparison to extremist liars.

The Only Thing I Know For Real - It has way less synths than other boss themes because Sam is not very cybernetically enhanced compared to other bosses - only his right arm is cybernetic, other than that, he's purely human, just in a powerful exosuit that makes him comparable to cyborgs. In extreme contrast, we have...

It Has To Be This Way - It's almost entirely made up of synths because Armstrong has so much nanomachines that he's even stronger than a cyborg - that, and nanos are commonly used in cyborgs, as indicated by Sam's dialogue in Mile High before Monsoon's fight. Because of this, Armstrong may as well be a cyborg because of his nanomachinery and sheer strength.