You can kill everyone, so at some point you have to make the call of: is it worth murdering every living creature in the game so you can level up a little earlier than you would otherwise?
For me, I prefer to just play out conversations as my character would. If it starts a fight, then I fight. If it convinces them to join me, great.
You get xp for completing missions, and there a plenty of fights with deserving creatures for you to kill and get xp from.
You can pretty easily hit level five before leaving the first map after the tutorial and that is like 1/15th of the game. Plenty of xp to be had without being a murder hobo.
