Personally I have always just disregarded the first law, seeing it as kind of stupid. There is even a WOJ if I recall basically stating that not all the magical laws actually make sense ethically. I believe law one is mentioned.
The law always seems stupid to me in general. Magic is just a tool like any other. If I can kill with non-magical instruments, why should magic make a difference. I know the WC justification is that Magic used to kill somehow corrupts the end user, but this would seem to be self-evidently preposterous. For starters, even if there were true in this manner, it would render the use of magic indirectly unjust as well in some circumstances. If I make a magical sword specifically to kill my enemies, I have still invested my magic in a death implement.
But even aside from that, its philosophically nonsense. As I understand it, they dont like it because it has some kind of negative affect on its user because the user had to invest their "self" in the killing in order to produce the magic. If this is the case, then only killing for unjust malicious intent would have said affect. If the wizard killed justly, then any adverse affect should be essentially nil since the justification of the killing would be a reflection of the "self" applied to the magic. Additionally, this law seems to imply that this presumed negative affect would affect the person free will. If a person who killed justly is still negatively affected, it should still not matter so long as they can theoretically exercise choice. Not to mention that you could equally stupidly apply this logic to non-magical means of killing. And if this were true, the whole WC and Dresden would have to step down due to "killing stress" or some nonsense.
Two Counters to this. First, at least as I see it, one of the themes of TDF is that actions have consequences, and those consequences are often entirely unrelated to the intent behind the action. Put another way, ethics and morality in the Dresdenverse seem to be closer to Deontological systems than Utilitarian ones. So, while the act of killing may at times be a necessary bad act, within the context of of this universe it is still an inherently bad act, and so it adversely effects the user. And it isn't a bad thing becasue it adversely affects the person who does it, it adversely affects the user becasue it is a bad thing. As for why this doesn't affect apply to non-magical killing, well, that's becasue magic. It takes an effect that already results from killing (look at the kind of PTSD suffered by soldiers/police/etc., even those involved in ultimately justifiable or at least necessary violence) and cranks it up to 11 becasue the supernatural nature of the event gives that trauma a direct line to your soul. Same general type, just massively increased scale.
Secondly, and I'll point you to what Luccio said in Turn Coat. Basically, the White Council and it's Laws aren't actually about right/wrong, good/evil, or morality. So, even if your argument did apply (which I don't think it does, at least not in context of the Dresdenverse), it doesn't change the fact that killing with magic is way, way easier than killing without it. sure, when you look at things like Nukes or chemical weapons there are mundane ways of killing on a similar scale, but the are not generally the sort of thing an individual would have access to. On the other hand, think about the amount of death someone like The Merlin, Ebeneener, Morgan, or even Harry could do if they went off the deep end and just started killing people. So, the White Council takes a hardline at restraining that particular use of power, above and beyond the morality of the situation. This also means that mundane killing, completely apart from morality, is simply not their department.