It's not just a stance, it's laid out in the Book of Vile Darkness resource, page 8, regarding Damning or Harming souls.
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Edit:
You're still only harming them physically when killing them normally when they get to Ravenloft. Where their soul gets reconstituted if it is trapped in Ravenloft is a sin for the dark powers to work out.
If I recall correctly, any Fiend that is corporeally destroyed has to have a phylactery macguffin to reconstitute itself as a possessor spirit, then it has to successfully transpose with a mortal's body to reform physically. If they don't have the macguffin to keep their spirit intact, they're irrevocably destroyed as if you summoned them on their home plane. Plus Gate, as I said, is a calling spell. Which automatically applies the above 'destruction of the soul' quality to the summoned Planar being if they die.
Damning or harming souls which are not evil outsiders.
Permanently killing a fiend is not an evil act, on the contrary - that's why Paladins are warriors and not pacifists.
The book never specifies anything about alignment impacting whether the destruction of a soul is an evil act or not, therefore it is a safe assumption that it holds all souls as sacrosanct. Every soul is in abstract a component of the multiverse and destroying it is, in a way, indirectly harming the multiverse. Most Fiends don't even consider the option of utterly destroying someone's soul; corrupting and torturing it, sure. But destroying it, nope.
D&D morality and the alignment system is inherently broken however; it's why whenever I run games I bin the entire system. Instead, Paladins are beholden to their Deity above all other concerns. Granted, there will still be that overlap of acting virtuous, but now the relationship makes more sense. Also implementing a d100 system for a Deity to notice you blaspheming or whatever opens the door for the Paladin to become a Blackguard in a way that makes a little more sense in my eyes; the Paladin starts killing bandits on the road that have surrendered to them, knowing they'll just attack other people if left alive. Their Deity fails the diceroll and doesn't notice, so the Paladin mistakenly assumes it's justified because their God would say otherwise.