I think the idea is that while hemalurgy itself is evil (tearing apart someones soul and killing them is pretty bad), the use of the spike afterwards is the morally neutral part. The deed is done, the evil is committed, the use of the spike is not inherently evil at that point. That said, hemalurgy I think comprises both those parts, so it's certainly arguable. I suspect he meant it's no more evil than Allomancy or Feruchemy, it's a tool. However, it's hard to imagine using it and not killing someone/shredding their soul, so who knows. Maybe if they consent?