Very possible, although this could cause a lot of bugs. You did get a notice about the broken quest every time you killed an NPC, I presume? You would have to use the console to fix that too. Check "Console Command Tutorial" on UESP.net if you want to have a go.
I would recomend re-loading an earlier save. If you don't have one, well, sorry. You really shouldn't kill off NPCs you don't know much about, at least not without saving first. :brokencomputer: