@sephanor: That is not normal, at least as far as the amount. I think it's supposed to be about 10% of their next level, and if you got 800k souls, that means you invaded a diehard who just happened to die. It's possible for this to happen, but you seem to have lucked out.
As far as whether to use them, well, that's up to you. With that many souls, you could definitely break the difficulty of your first playthrough. I'd use like 100k and then die to get rid of them, but hey, it's up to you. I'm sure many people would kill for that many souls.