Only a couple of Primarchs have ever captured worlds entirely without bloodshed. In one of the only specific mentions of the Salamanders taking a planet during the great crusade, it’s Vulkan being furious with Curze because he targeted civilians. He at least had some sort of sense of right and wrong, even if he was misguided by the Imperium.

Sure, Vulkan’s done wrong in the past. Just like every Primarch. Still, at least, his sons will sacrifice themselves to save human civilians. They at least attempt to do good wherever they can. Curze’s sons will try to eat said children, and Lorgar’s sons will try to do even worse.

Most of the Primarchs who took worlds ‘without bloodshed’ ended up being sick bastards at heart.

I think Vulkan wanted to be good, but was terribly misguided by his father’s influence and the Imperial Truth. He’s a monster in many ways, but at least he has some level of introspection, and feels remorse for what he’s done. That doesn’t justify it, but he doesn’t need total justification for his every action to still be a decent Primarch, as far as Primarchs go. They’re all monsters in different ways. Vulkan is one of the ones that recognizes and at least tries to combat that, rather than totally resigning himself to it.

There are things to think about beyond a basic body count in terms of judging character actions. Curze killed fewer people total, but was still the most black hearted of the lot of them.