Also, a note: To do magic is to believe in what you're doing with magic, which is why someone who kills with magic--or breaks any of the other laws--is tainted the way they are. However, the warden swords were enchanted by someone else, meaning that their magic isn't native to the wielder and that distance might help with any issues with the First Law.
Second, the magic on the sword just makes it a better sword; it's a much more passive sort of action than even a gust of wind blowing someone off of a high rise is. As intent matters, remember that a sword is a sword is a sword--a big, sharp thing that works by the pointy end going into the other guy--whereas a spell comes from the will of the wizard. In one case, you're killing someone with a sharp piece of metal that is a sharper piece of metal thanks to some magic, but the metal is still doing the killing. In the other case, an expression of a raw element given form by your will is doing the killing. One is more personal than the other.
And third, just to switch sides a bit, perhaps the twitchy sword fingers that are endemic among the Wardens are related to this; maybe they're getting just a wiff of bad mojo off of the swords when they kill with them; not enough to qualify as First Lawbreaker, but enough, especially when combined with the destruction around most warlocks, to nudge them in the direction of "When all you have is a hammer, all your problems begin to look like nails."