Allow me to go all Ne deep here, somewhat. So, I've seen both the new and old versions of the FMA series. One thing that always struck me is the concept of the deadly sins. Now, the use of alchemy and and the overall plot to defeat Father and for Ed and Al to get their bodies back... it all revolves around deeper life lessons/morals. This is especially evident in the end of the series when Ed is faced with death at the gate of truth, and chooses to sacrifice truth/knowledge to be with his friends (a little something I believe many people would be wise to pay attention to).
When it comes to the deadly sins/homunculi, three of them have particularly interesting titles: Lust "the ultimate spear", Greed "the ultimate shield", and Wrath "the ultimate eye". My interest is in why those characters , those sins, were paired with their particular powers. Here are my thoughts:
Greed is the ultimate shield for reasons that tie directly to the nature of greed, and the types of people who take it to excess. Greed, the unabashed desire for everything material, is present in all of us to some extent. But there are those who take it to the furthest extent, who we would often look at and pity as being shallow or empty inside, because they seek more money, more power, more women, etc., never being satisfied, and never finding anything of greater, spiritual substance, or love. Such people often put up quite a mental defense, and ultimate shield if you will. They are hardened, numb to the pain of living a life searching for love, god, etc. All those "higher order" values. They go through life not caring what anyone thinks, mostly caring only for themselves. A man who cares nothing for anyone else, only his own desires, and who will do whatever it takes to get what he wants is truly impervious to any pushback from society, so long as he sticks within certain limits. Beyond that, I'm sure many of you know those kinds of people who wear their heart on their sleave, and truly care about their friends and desire family and love, but act hard/tough as a way of shielding their own heart, and seek everything under the sun to fill that void left inside. Such is the essence of greed's character and his power, the ultimate shield.
Lust. She's hot. She might not lust after anyone herself, but her power is seduction, inciting lust in those who see her. Like an attractive woman who has grown tired of endless male suitors and has a chip on her shoulder, she she uses her beauty as a weapon. And thus, she is "the ultimate spear". Is there any greater weapon than seduction, and sharper blade than love, which pierces straight through the heart without fail? That is the ultimate spear.
Wrath, the ultimate eye. Anger, hate, WRATH. It gives us the impetus to act with speed and vigour to strike down what offends us so. When one is enraged, it's a whole nother state of consciousness. Your vision becomes narrowed, and all you can focus on is the target of your anger/hate, and how you will eliminate them from the picture. Truthfully, it can allow us to see into the future, a future where we've removed the thing that pisses us off. In wrath, we are singularly focused on the end goal (and the same can be said of other states as well). Thus, Wrath has the ultimate eye, the eye that sees the future.