[ Spoilers for Changes ]
Everything sinister is out of the question. Everything born out of pure aggression is out of the question unless it doesn't serve the protection of innocent mortals. Generally it seems to come down to doing rightful good. When we look at Harry, battling the forces of evil seems to be perfectly OK with the powers of heaven, but I guess Harry may be some kind of special exception when it comes to his soulfire... There should be a good reason why the PC can wield it I'd say...
I can't see anything in the source books saying that you absolutely can't use soulfire in an act of pure aggression, or even out of malice. And Harry actually seems to do something very close to that in Changes, when he lashes out at the Red Court vamps in his office building.
At the risk of pissing people off for going off on another tangent which deals with stuff outside the rules of the RPG, I feel I should mention that the concept of soulfire as sponsored magic may not be entirely accurate, at least from what we've seen in the novels. This is just my own guess here, but, while Hellfire is definitely sponsored by a Fallen angel, Harry's soulfire seems to come entirely from within himself. He uses his own soul energy, his own magic, and his own skill to cast his soulfire spells. Uriel may have done something to give Harry whatever crucial piece of understanding that Harry was lacking in order to link his soul with his magic, but Uriel isn't the one supplying Harry with soul energy. Or anything else, as far as we know right now. Harry seems to be the one supplying all of the ingredients for the soulfire, with no further input from Uriel or anyone else.
With other sponsored magics, like what the Faerie Knights have, or Outsider possession, or Hellfire, the sponsor provides the necessary magical energy and sends it to the person they're sponsoring. In Harry's case, though, it's all down to him, so I don't see why a Higher Power would be getting involved. I mean, Uriel apparently got involved initially when he created the link between Harry's soul and Harry's consciousness, or his soul and his magic, or whatever it was that Uriel did; but, beyond that, I don't see any evidence of further involvement.
And if that's the case, then there may not be any direct influence by Heavenly powers while using soulfire. In fact, thinking about it-- if someone could create soulfire using their own soul, like Harry can, then I don't see why they couldn't conceivably change that soulfire into Hellfire someday. There might be a reason, like some fundamental difference between mortals and angels that causes an angel's soulfire to become Hellfire when they go bad, but we haven't got any clear evidence of that yet. Barring any such evidence, it seems possible that someone could go far enough into the Dark Side that their soulfire turned into Hellfire.
That's just a wild-ass guess on my part, though. I may be completely wrong about this possibility.