I walked into a cave and there was this one guy standing there (I won't say his name for spoiler reasons, it was not umbra) and I killed him and now I'm panicking thing have I just messed up a quest I could of got later on?

Thanks guys.

I want to know too. There's a shopkeeper who has a ring of sneaking with a 0% chance of pickpocketing. I want to kill him for it but don't want it to come back to bite me. I know there are some characters who won't die tho. They just lay there saying enough when you try to kill them. So I'm pretty sure you can't kill vital NPC's

Quest-essential NPCs can't die.

Not sure if its quest items, but I found a random cave I cleared out, and after I got a quest to go into it and college an item from it.
When I got the quest, there was an option say OH YOU MEAN THIS..... and I got to complete the quest.

So my guess is, if you kill someone that has a quest where you need to kill them, there will be an option, oh they are already dead.
OR you will just get the quest, then have to talk to them right after you close the window to complete the quest.