Hm. While that could be considered a (very) iffy first (or fourth) law violation, I'd say it's actually a zeroth law violation: A violation of the trust between player and GM. If the player is going that far out of their way to make certain their wards aren't going to violate the laws of magic, and the GM decides that "oh, oops, you killed someone anyway!" - well, that's just not right. It'd be like telling a PC "Here, have a fate point for hexing your friend's radio" - and then, once they accept it, adding "Oh, and that also hexed the pacemaker of the guy you were trying to save. Too bad."
For the record, I wouldn't consider that a first law violation anyway. There were multiple acts of free will involved (from the victim if nothing else) between spell effect and death. For an example, suppose you used magic to light a campfire. The fire, once lit, is nonmagical, and then some idiot trips over the firepit, falls in, and gets himself burned to death. Is that your fault? No. You'll probably feel horrible about it, sure, and it wouldn't have happened if you hadn't lit that fire, and the wardens will investigate because you could have been using an entropy curse... but in the end, the wardens will pack up their bags and go home, because there wasn't an entropy curse, there wasn't any direct cause and effect between your action and the death, there wasn't any intent to kill, etc.
Now, on the other hand, you could make a very interesting story out of having what you described happen, and then requiring your PCs to figure out what was really going on and track down the warlock who put an entropy curse on the victim, all before the wardens showed up and started killing off PC wizards...