Actually i think soulfire is completely different from magic. It feeds into to magic rather than the other way around. I bet the angels don't use magic, because remember when michael told harry that magic was evil/unholy
Bob describes soulfire this way:
“So what you’re saying is that this hand construct was made out of my soul,” I said.
“Your soul and your magic fused together, yeah,” Bob said. “Your soul converted into energy. Soulfire. In this case, the spirit energy drawn from your aura right around your right hand, because it fit the construct so well, it being a big version of your right hand and all. Your standard force-projection spell formed around the matrix of soulfire, and what had been an instantaneous exertion of force became a long-term entity capable of manipulation and exertion to the same degree. Not really more powerful than just the force spell, as much as it was more than simply the force spell.”
There are two different interpretations of this, as far as I can see. The first is that soulfire is made directly from the energy of the soul, and then the magic forms around it. And second, that soulfire is itself the combination of magic and soul energy, so that any spell performed with soulfire can essentially be called a soulfire spell, rather than a magic spell laced with soulfire.
The first description fits with the last part of Bob's description, but not as much with the beginning. The second description fits the beginning what Bob says, but seems harder to understand when combined with the last half of his description.
In my opinion, the way Bob begins his description seems simplest and most likely to be what he meant-- soulfire is "your soul and your magic fused together." When he says later that the force-projection spell formed around a matrix of soulfire, it sounds to me like he's saying that the
spell itself took shape around the matrix of soulfire, not that the
magic alone took shape around the soulfire. Which seems like it would mean that when you use soulfire, you kind of use the soulfire in place of magic, since the soulfire already has magic fused into it. Whereas normally you would use magic to cast a spell, with soulfire, you fuse soul energy and magic together to create soulfire, then use the soulfire to cast the spell.
At least, that's what it sounds like to me. I could be wrong.