This is my interpretation of what was going on in Ed's head during the last scene of episode 48. I do not claim this to be the actual intended message. Also, I haven't completely finished the series yet, so if there are any major canonical errors, please let me know.
Disclaimer: Fullmetal Alchemist is the property of Square Enix and several other companies that I can never remember. I claim no ownership to the show or any of its characters, places, or ideas.
Not a word was spoken as the old car ambled along the ancient streets of Central. All there was to say had been said already by Edward only a half hour before. Any other words would be pointless at this time; all three of them knew it. Ed was rarely left with nothing to say – he had always had an opinion about every topic that was brought up – and it was an odd experience. By now, though, it mattered little; the end was in sight, so close that the young alchemist could practically reach out and touch it. Each of them, even Winry and her grandmother, stood upon the brink of an immense abyss, while the land behind them blazed with a crackling wildfire. Step backwards, and you burned alive; step forwards, and you fell to your death. No option existed where they would all come out in one piece.
Glancing first at Mustang on the seat beside him, and then at Hawkeye behind the wheel, Ed felt a wave of despair wash over him in a merciless flood. "Just figures, doesn't it?" he muttered savagely. "We finally reach an understanding right when we're about to die!" Mustang glanced at the boy, having heard his murmured outburst, but said nothing. He obviously needed his space at the moment.
With a sputter, the car creaked to a halt along the curb, signaling Ed's drop point. Sighing, the lad climbed out of the car and dropped down onto the sidewalk, shoulders slightly slumped. So this was it. The end of it all. As soon as this car left his line of sight, he was completely on his own. Al could expect no help from Mustang's squad – Ed would have to carry that task alone. He never thought he'd see the day when he'd prefer to have Mustang there to back him up, but then, ever since he had transmuted his mother, life had brought along more bizarre events than that. Still... it was weird.
Fullmetal stepped away from the car and prepared to take off, but to his surprise, he heard the car door open again, and saw, from the corner of his eye, Mustang climb out. Turning, Ed put up his best tough-guy scowl and snapped, "What now?"
"Keep in mind what you're facing up against," the general replied stiffly. "If you start acting carelessly, your brother is as good as dead. Never, under any circumstances, are you to underestimate these people. Do you understand?"
Right then, Ed had a very nearly overwhelming urge to punch the general in the face. If he had been even one year younger, he would've given in to it; but luckily for Mustang, his experiences over the past year had gained him a lot more maturity than he had previously possessed. Still, Mustang's "Captain Obvious" statements really got on his nerves. "Yeah," he sneered, "I think that I know the stakes a bit better than you do."
Anger flashed in Mustang's dark eyes, and he looked as if he were about to spout out some scathing retort, but then, something softened. Backing off a bit, the Flame Alchemist dipped his head politely. "Forgive me, Edward. I fear that I can be a bit overprotective of you and your brother sometimes. I apologize."
Ed stared at his superior in undisguised shock. That... was the kindest thing that he had ever heard Mustang say – which was sad actually; he wasn't exactly known for being a "nice guy". And judging by his expression, he was very unaccustomed to it; his feet were shuffling awkwardly, and his face was twisted up in a grimace, as if he were disgusted with himself that he had said that much. But still... he had said it, and his tone had suggested that he meant it.
Had Ed been wrong about Mustang this entire time? Were those secrets he'd kept, and those lies he'd told, really for he and Al's protection? Now that he thought about it, he really didn't know the Flame Alchemist at all. Sure, he knew that he'd fought in the Ishbalan Rebellion, and that he had done some pretty awful things, but about his family and hometown, he knew nothing. And it wasn't even that the man had tried to hide it; Ed had just never asked. Actually, he had never tried to get to know the general at all.
Except... he recalled what Winry and the librarian had said, about Mustang fighting for Hughes, and life that he hadn't been able to lead. How the man had given up everything for the memory of his closest friend... even his reputation... even his freedom.
Maybe the two of them weren't so different, after all.
And maybe... if things had been different, if there were no Homunculi or Philosopher's Stone or Ishbalan Rebellion, the two of them could've been friends. After all, Ed knew deep down that he wasn't really a bad guy; no worse than he himself was, anyway. Perhaps, if they both survived this...
But what were the chances of that? They were both standing between the fire and the abyss. They both were headed no doubt to their deaths. There would be no second chances, no fix-ups, no do-overs; not for them.
Mustang lifted his hand to his forehead, about to salute, but stopped halfway. Changing his mind, he extended it out to the younger man. Ed eyed it briefly; but instead of shaking it, as the general had no doubt intended, he gave it a friendly pat. The Flame Alchemist looked surprised for a moment, and Fullmetal took the opportunity to flash a rare smile – not one of his typical only half meant respect, but one of comradery, of friendship. Mustang smiled back, and for one second, everything was alright.
Then, at the same time, they both grew serious again. The general stepped back and climbed back inside the car, locking eyes with Fullmetal as the door slid closed. Ed held his gaze as long as he could, watching the vehicle drive away. It wasn't until it was out of sight that he finally turned away from the scene.
It was time.
