A npc pissed me of so I went into combat mode, saved the game and then shot the npc in the face, just for fun. I then reloaded again but suddenly everyone was hostile in town. Is there a solution to this?

If Fallout's hostile programming is similar to Wasteland, its spiritual ancestor, just the act of entering combat mode may have been enough to trigger the town to go hostile towards you. If this doesn't usually happen, I would say you found a bug and unless you archived previous saves you may have to either A) Start over or B) Kill the town.