"Whether you’re using magic directly to rip the life out of someone, summoning up force or flame to kill, or even murdering someone without magic and then using the energy created by the death to power a spell, you are breaking the First Law of Magic." (YS235)
Mixing mortal death and magic is bad, m'kay?
Addendum: If a wizard goes on a murderous binge, quartering babies with a hatchet and decapitating the entire populations of small towns, this is perfectly acceptable -- at least so far as the Laws of Magic are concerned -- as long as magic isn't involved on either the dealing or recieving end of those deaths. This is not to say that doing so will win friends and influence people, and it may well be that the Wardens will be sent after you just to get rid of a threat to the welfare of the Council, but it won't be because of a violation of the Laws of Magic.