When he and his brother tried to use alchemy to get their mother back, it went horribly wrong. Edward, the older brother, lost his left leg, whereas Alphonse lost his entire body. The result was a horrifying monster, far from their kind mother. Anyways, Edward acted fast, because he only had a small window of time in order to get his brother's soul back and attach it to something. So, using alchemy once again, he gave up his right arm in return for his brother, who's soul was then attached to a suit of armor, seeing as though that was the only thing there useful as a vessel at the time.
So when the thing in my last post is said to him...