See, I get the concept of "Warren did bad things with a soul, Willow did bad things with a soul." But I think it's a bit... simplistic.
Or maybe, it's a bit of an easy rationalization for those who *want* to see Spike go kickass, S2 neck-chomping evil again.
Yes, Willow and Warren -- and Rack presumably -- and Saddam Hussein, did bad things, with a soul. Could be spurred to do evil, even with a human soul.
But those souls? They were along for the ride. When Warren went bad? When Willow slid further and further over the line? Those souls were right there with them. Being corrupted.
Spike's? Worst case, it's brand new. Most likely, given his ramblings, it's William's soul. It wasn't *there* for everything evil Spike's done. Wasn't even there for the assault in Seeing Red, when he was theoretically acting more out of human madness than demonic evil. The William part of him? Seems appalled by what he's done. The most William's soul was ever corrupted was... what? Two seconds of looking into Drusilla's eyes?
I'm not saying Te's story, for instance, isn't plausible -- William being a creature of great weakness, giving in to power over himself and lack of confusion? Utterly possible. We don't know. We've only seen him be...mediocre. According to himself. Spikeself. Though imho, confessing the truth to Cecily about his feelings for her was actually fairly brave, in his own Victorian cosmology.
But the argument by itself, that's floating around? That the soul is *no* reason to assume Spike wouldn't go back to killing? I don't buy it. I don't buy it on a logical angle, not on an Amy Can't Bear Spike To Be Evil angle. William's soul, as it was when he died? *Nothing* like Warren's soul. Nothing like Willow's at the point when she went over the edge.