If you violate the first law, even in self defense, you still have the taint. This is evidenced by the kid in SF who could see the taint of HWWB on Harry.
That mark on Harry was a scar from contact with the Walker while it was pursuing him, not the later action of killing Justin in combat. Though given the flashback in GS, the Walker seemed to want to set him on a path to go after Justin, so it's a fine distinction.
Yeah, Hannah Asher had reason for her initial killing, being raped for heaven sake, but she didn't surrender to the Wardens at that time. If she had a good advocate she might have survived under the Doom. ... So it is understandable that Hannah ran from the Wardens, but at the same time was she ever able to check her slide into warlockhood? It isn't totally clear whether or not at the time of Changes when her friends died when their vamp halves died that she wasn't already a warlock.
She was a warlock for killing the Wardens who tried to arrest her, at least. But there seems to be variation in how high-functioning warlocks can be. The paradigm is that rationalizing that it's right to do an act of dark magic warps the caster so they're more likely to do the same thing again, right? So someone like Hannah (or Harry himself) who kills in self-defense is changed by that experience, but I think it's specific to responding to future threats with wrath, rather than a general temptation to burn everybody for shits 'n giggles. Harry even recognizes that tendency in himself - among other mentions, one of the significant realizations in Ghost Story is that he's tended to embrace anger when threatened as an alternative to fear, and he can't really do that when he's watching his friends in physical danger but can't intervene himself. He just doesn't connect those temper issues to his initial experience with killing Justin, but I think that's where they originated.
It's a qualitatively different personality change from someone who kills for personal gain, or in anger that's not connected to a direct threat to their own safety (e.g. the young Korean warlock Langtry used as an example). The option of leniency in self defense cases probably exists because there's more of a chance of rehabilitation for those who have become wrathful when genuinely threatened than there is for those who have changed to believe in using their magic to initiate aggression. I don't think it was an accident that Ebenezar kept Harry largely isolated to a safe environment on his farm (aside from the one encounter with teenage bullies in town, where Harry remembers just staring them down because he knew they weren't a real threat - in hindsight, I suspect Eb was treating that as a test).