Your understanding of fiend deals is not quite full.
A pact can even be signed with all the benefits and no downsides.
I read for example a backstory where PC's father, a powerful Warlock himself, summoned and imprisoned a fiend, forcing it to sign a pact with you.
https://www.google.com/search?q=warlock+fiend+pact+backstoryAnyway, this is not an open world game. Larian wants to tell you a story and just like in DOS2 it will have a few endings, but I wouldn't expect real impactful decisions behind most encounters.
If you manage not to kill certain NPCs you are very likely to see them again in Baldur's Gate, but will it have a big impact on the story itself?
