No honour in killing the defenseless, but with it being Skyrim and all, your average townie should be a bit tougher than their Cyrodiilic counterparts. One journo said he attacked a barmaid, and couldn't kill her due to her stock of healing potions; rampages might not be an easy option at all.
Anyway, fantasy games are great, you can be an absolute saint, and still become filthy rich. That will do for me.
Oh...but it is. It doesn't matter if it's a Mark, to be assassinated or a random NPC. I try to kill each one differently.
Reverse pickpocket a cursed hood. Then cast a Command spell. They will stand there, with a big grin on their face. I stare them in the eye, as their health ticks away to nothing.
Command spell, and lead then to a cliff. Then push them off.
Remove all the food in the house, and fill their plates with chokeberries. Then set in the corner, wait and watch.
In all cases, I arrange the bodies, just so and leave a Daedra Heart on their chest.
Maybe your right...honor plays no role. But a calculated, creative and slow death is very...satisfying.