I'm just wondering if it's possible to mess up quests before you get them.
I just raided Rockmilk cave but i had a feeling it was going to be quest related which is it.
Will i still be able to do the quest even though i've killed anyone?
Yes you can mess quests up before you get them. In general, avoid killing anyone who has a name of their own. "Bandit" "Highwayman" "Guard" and that kind of descriptive name means they normally respawn, and are always safe to kill. There are a few cases where the NPC's get themselves killed without you doing anything, by falling off a bridge or getting into a fight while they're on the road between cities. Most of the folks who can do that aren't quest-related, but one or two are.
All the inhabitants of Rockmilk cave are generic Bandits or Marauders and can be killed again every three days for fun and profit.