At the basis of all transactions, there is the trade of like things, the Theory of Equvialent Exchange that Alchemists treasure so reverently.
Kindness makes kindness, love makes love, but hurt makes hurt and all things bring revenge in the long run.
Greed had not been alive so long that he hadn't felt the desire to live just one more day, the want to be a little bit more, the pull of things left undone and the dream he could have it all. Each day he lived only increased those feelings, those basic instincts of survival until living, being and wanting was almost all he knew how to do.
His sort of adoption of the chimeras was unintended; they were supposed to be tools in the beginning, possessions that would propel him along and could be lost and discarded in the end without thought. Slowly though, they became more. Whatever was closest to family love his non-exsistent heart could feel, welled up when they came back after every job he sent them on, and slowly Greed could not picture life without them.
He gave them freedom, life, and in turn, they gave him theirs.
In the end, when his death was inevitable, he thought of them, how he had never properly thanked them for all they did. The soul that did not exsist left his body and he was gone.
Keep faith with me, and I will keep faith with you.
This is currently un-betaed and thus not my best. It seems kind of confusing, even to me, so I won't be offended if you say so. The last line is reference to Martel/Marta's attempt at avenging Greed and the others. He sort of saved her life so she tried to avenge the loss of his.