To discuss this by way of example:
If you're a dirty rotten cheater, and you mute/freeze/stun the 9th level cultist wizard outside the tutorial dungeon - and kill him before he has a chance to speak - then "Protect the Stone; I'll handle these vermin." never triggers.
Kill his 5th-level guards and carry on: nobody has a stone, subsequent conversations and events still play out with no changes.
There's no reason for Larian to cover off this case. Anyone who does it is already deliberately working outside the story, and has no kick if they aren't accommodated.
Killing Bellegar may work out the same way. In his case, it doesn't even really break anything: do you expect somebody like Bellegar to stay dead? Really?
