Man, I thought I played the game in an unconventional way that introduced a few quirky interactions, but this is a whole different level. My hat's off to you for the weirdness factor, and I'm all for respecting player choice and emergent gameplay and all the rest of it. But I think at a certain point a developer is justified in saying "we're not gonna bother trying to anticipate every possible weird configuration of outcomes that a player could engineer." Like, it's a ROLEplaying game. You are playing a ROLE. Can you come up with any non-video-gamey reason for your character to act the way they did? You had two possible leads on how to solve your problem: you killed one of them, and you killed all of the other one's friends, and you did it in a really strange order.
You did what you did because you decided you, the player, didn't like the outcome of not getting Halsin as a companion. Leaving aside that you really should have been able to guess you were foreclosing on that option by killing all his friends and then letting the goblins slaughter a bunch of civilians, the answer is to go back and load an old save and do it differently. The answer is not "well I'll just kill even more people."
I do agree with you that it seems like broken reactivity if Wyll and Karlach don't leave you in this scenario, because not only did you side with the goblins and doom a bunch of civilians to die, but you also eliminated their only line of defense first. But again, this is such a weird way to do Act 1. And I'm a guy who often does things in a weird order. My dumb brain read a throwaway line in the quest journal as suggesting that I should do things in what turned out to be the very incorrect order in Act 3, which resulted in me missing some cinematics and not witnessing some events and not getting to intervene in some things. It was such a bummer that I bailed on the playthrough, at least for now. But even that wasn't as wild as what you're describing here, and the reactivity wasn't broken, it just wasn't as fun. I can't stress enough that I understand the struggle of being a weirdo who plays games weird, but some things are too weird to expect a developer to plan for.