Murder in California requires that the killer acted with a state of mind called "malice aforethought," which is generally referred to simply as malice. Malice may be either express or implied.
Express malice is where the defendant intended to kill. There is no evidence that Dr. Murray intended to kill Michael Jackson. Indeed, it appears that Michael Jackson's death cost Dr. Murray a lucrative contract of $150,000 per month to be Michael Jackson's personal physician. Dr. Murray thus had every incentive to keep Michael Jackson alive.
Implied malice occurs when: 1) the defendant intentionally committed an act, the natural consequences of which were dangerous to human life, 2) at the time he acted, he knew the act was dangerous to human life, and 3) he did the act with a conscious disregard for human life.
There is a stronger case for implied malice than express malice here. The combination and amount of drugs found in Michael Jackson's system have been described as extraordinarily high. However, if Michael Jackson was an addict who had developed a high tolerance for these medications, and if Dr. Murray had been giving Michael Jackson similar doses in the months prior to his death without issue, he would have a strong argument that he did not act with a conscious disregard for human life. Similarly, if Dr. Murray was attempting to wean Michael Jackson off the more dangerous drugs, like Propofol, as he claims, he could argue that he was not acting with a conscious disregard for human life.
However, such arguments will have a harder time overcoming the "criminal negligence" standard for involuntary manslaughter. (See posts from yesterday and the day before for more information on criminal negligence and involuntary manslaughter.)
Sometimes overcharging a case can backfire and lead to an acquittal (like some suggest happened in the Casey Anthony case). The DA's office likely charged conservatively to have a better chance of success, and the charge of involuntary manslaughter is really a better fit for the evidence here (as discussed yesterday).