Remember Harry's line: You can't do anything with magic you don't believe you can do. Killing is a part of that (even accidental killing; in retrospect, that eats into you and changes you too, though perhaps not so potently as doing it with intent does). Believing you can create a weapon is one thing. Believing you can use that weapon to do harm is quite another.
Though even there I'm shaky when the weapon is mundane, or used mundanely like a sword-stroke and so forth, since it's really about using *magic*; the reason Wardens carry those swords, supposedly, is so that they won't violate the First Law when they execute a warlock.
But here's the other thing: this is all opinion, from me, by intention. The subtext of the Laws discussion in the RPG is not "and this is how it is always with no wiggle". It's meant to say, how the Laws are interpreted -- both in-character as a body of Law that the Wardens enforce, and in-system as to when one is required to take (or expand) a Lawbreaker ability -- is something to be explored as you play the game. It's a journey, and as your game travels along that path certain themes should emerge. One game's themes might point away from the idea that the Laws-as-enforced-by-the-Wardens-matches-the-Laws-as-enforced-upon-your-soul; another's might insistently point towards it, suggesting that the Laws as laid down by the Council are Laws Of The Universe that have been uncovered and codified.