I've been thinking about this a lot lately. It's a really hard question to pin down, mostly because Ed and Al do not talk about what they did. They'll say, brother, I'm sorry for what I did to you—but do they ever say, hmm, this and this is why we fucked up? Ed does talk about it a teensy little bit in episodes 1&2, but he couches it in the mystical religious terminology that Rose will understand, and I still don't think that means he understands what he did wrong. It's one thing to say, Well, this is God's territory, and God gets pissed when I mess in his sandbox! But why is it God's territory—that's what I feel Ed just doesn't get.
Clearly, Ed feels regret for what he did. But so does a kid who gets caught with their hand in the cookie jar. Why do they feel guilty? Because they got caught, fucktard, not because they realize they shouldn't eat cookies before dinner. I think (and this is just my own personal opinion) that Ed, while he looks at the remnants of his failed experiment every day and does feel terrible guilt over the fact that he messed up so badly, and while he feels terribly that he transmuted a monster instead of his mother, still does not quite understand what he did.
The issue, to me, is a lot about abuse of power. You see it time and time again with alchemists. Why did Shou Tucker do what he did? He says it himself—because he could. Not to further scientific resesarch, not to save little babies, not to help his family: he just wanted to because the power gave him a rush. I imagine Hohenhime is a bit of the same if he created the Sins: why is he so interested in human alchemy? He just wants to push the limits. He wants to see if it really can be done. He wants to see if it'll work.
In 27, Izumi says why you should not revive the dead. Because it just isn't right. It disturbs the natural order of the world; the world needs death, after all. In the Iliad, I think, there is a very interesting section on Zeus and the Fates. There's a bit of a debate in classics over whether or not the Fates are the supreme beings of the world, whose power cannot be trumped even by Zeus, King of the gods. In this section in the Iliad, Zeus is looking down on the Trojan War and seeing that his son, his child (and Zeus had many, but he clearly holds affection for them all) Scamander is about to be killed. And he lifts his hand to save him, because he can't bear to see his son suffer and die.
And who else sweeps in but Hera, with what I imagine is the smuggest expression on her face, and she basically says to Zeus, You wanna do it? Fine. Go ahead, go ahead. But remember—every one of us has children down there who are fated to die in this war. And we could all swoop down to save them from their deaths. But what happens if we do? Total and utter chaos. Don't abuse your power, my husband.
And Zeus doesn't. He settles back, and his son is killed in battle. And so are most of the children of every other god who are fighting.
Izumi thinks that human alchemy is simply a violation of the natural order of the world. Ed, though? Does he realize this? I've not seen him do so, not really. He looks at Izumi with resentment when she says that. For Ed, it's not a question of is it right that I do this—it's can I do it. Can do human alchemy, can I make it work?
If you had the power to revive your mother, would you? Or would you step back and say, No, this is wrong. It was her time to die. I won't abuse my power like this.
We looked at Shou Tucker with disgust when we found out that he had made a chimera out of his daughter and her dog. Why would we not look at Ed with the same disgust when he revives his dead mother?
I think Ed wouldn't try to revive his mother again, but I think he wouldn't do it because he'd be scared of what would happen. He saw what happened to Alphonse and he carries that burden every day. But I don't think he understands why it happened. And when he does, I think Ed will really have grown up.