My guy supports stormcloaks, will eventually join the companions. I'd like to start off bad and earn redemption, but he's an assassin not a goat sacrifice /blood drinker.
So are they hitmen-for-hire with no conscience or evil demon worshiping necromancing vampires?
Does it impact the world negatively? (run-down city, crime, people hate you, etc) If so, can it be changed by joining other factions, or fiddling with the console?

The Companions are more blood-drinkers than the DB, IMO.
As for Narfi, not to sound like a jerk, but he's poor so he probably stole something like Degaine and Brenuin.