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.
Just because the Council recognizes mitigating circumstances, does not mean that the universe will give you a pass.
That being said, Half-Ramps are not human. The fact that Harry "Pulled the trigger" would not have shielded him from the effects had he killed humans. But they were no longer humans at that point. What happened to them after the spell had it's way is of no consequence to the spell caster (which Harry was.) With that Curse, Harry killed Rampires, and Half Rampires. The effects of the spell destroyed the "Rampire taint" if you will.
Killing non-mortals is not against the first law, and Harry has no additional "taint" on him.
That being said, killing a living being, or beings in this case, DOES have an affect.
It's like when I was in college. Those 8:00 am classes were EARLY in the day. I knew that when I signed up for them. I also knew that the first time I missed a class, it was SO much easier to skip a second and a third and a fourth that eventually I might as well just drop the class.
It's the same way with other things, like killing. It gets easier the more you do it. Or it'll drive you nuts.
I believe that the Black Magic taint is separate completely from the WK Mantle.
That being said, feelings are feelings. Whether they come from the WK Mantle, or from an easing of the ability (to kill in this case) doesn't really matter. You have that feeling.
Could the urges that the WK mantle provides be goaded on by Black magic taint? I see no reasons why not. But What it think is more likely is that the WK mantle says Kill, and Harry is more likely to do it because he already has killed, not because a taint is making him do it.