I always think he tries to figure out other ways to be immortal so his friends can live with him. He is immortal, he has a "better" carbon body, why would he need Alphonse?
More mature Elrics would easily allied with that Greed. but the heck, the story...........heh it would be much shorter. They would learn all about Father conspiracy, well,they still would require more power force to do anything. Greed's forces arent strong enough. and they needed more holistic solution.