In a strict legal sense, the killers are the murderers. But in a moral sense, proponents of the drug war, including both U.S. and Mexican officials, are as responsible for the deaths of those 13 police officers as the killers are.
That's because the drug war is what spawns the drug cartels, which then go out and ambush and kill police officers. Or to put it another way, without the drug war there wouldn't have been a drug cartel and, therefore, those 13 police officers would still be alive.
Drug cartels are not the only negative consequence of the drug war. It has also brought into existence robberies, thefts, kidnappings, burglaries, muggings, and official corruption. For example, when drugs are made illegal, that causes black-market prices to soar, which then drives the addict to commit a robbery to get the money to pay the enormous price for his drugs. Or consider, as another example, the fact that law-enforcement officers, prosecutors, and judges are offered the opportunity to accept a bribe to look the other way on a drug deal or to go easy on a particular drug dealer.