As my time in Amalur wound down last night, I decided to do the only logical thing and murder everyone I came across. I managed to make mincemeat of most of a village of Fae, but then I encountered someone who had a quest indicator over their head. I couldn't do damage to them, and talking to them seemed to indicate that they were still regarding me as a "friend" and that they still wanted my help with a quest.
(This was despite the fact that I was repeatedly bludgeoning him over the head with a magical staff of burning things with fire that was soaked with the blood of their compatriots, but I digress.)
It seemed I was able to take out most of the "non-essential" NPCs in an entire town this way, and I wasn't once stopped by a guard or other law enforcement individual. Further, there was no reaction from the citizens spared on account of their relative immunity to my attacks.
What are the limits to my murderous rampages? Can I kill shopkeepers? Does it matter if they're "generic" villagers or they have unique names? Are there any side quest NPCs that I can murder?
Further, what are the consequences? Will I be caught and sentenced to 2 minutes in prison at some point? Is there some "faction standing" that I tanked during this?