Oh I feel this. There's been playthroughs I've started purely to be evil, yet I still end up feeling bad most of the time.
I think the only run through I've had where I actually felt like I did a good job "justifying" it was with my very old Nerevarine Telvanni Sheogorath-worshipping sorceress that I played through Morrowind-Oblivion-Skyrim with. In her case, she wasn't trying to be necessarily evil, she was just blunt, had little patience, was straight to the point and didn't consider how her words or actions may affect someone, as long as she could continue hoarding artifacts and doing nutso mage things, she was happy.
She also didn't understand why murder and thievery were illegal, if she saw something she wanted, she took it, she didn't get why the guards would be upset over it and was usually disgruntled with Skyrim's law system.
You mentioned Narfi, and that's one I remember giving a justification for on that play through. In my sorceress's mind, he deserved to be in a better place more suited for him, like the Shivering Isles, so off she sent him via a Lightning Bolt to the face.
I don't have any mod recommendations, but I wanted to give an example of how I justified it myself. I didn't feel as bad as I usually did with prior evil play throughs. :)