Trump is wrong here by assuming current government of Mexico wants any change. Since they took power, things have gone worse because they're Leftists who are allied with the usual Venezuelan suspects. The town where the Chapo's son escaped had a narco-affiliated as a Mayor. I doubt el presidente wants Trump's help and not only because he's close to narcos himself, but because his political closeness to Maduro and Chavismo makes him believe USA is practically the devil and they have no business within Latin America. If he complies with anything Trump wants, it's out of fear. He really doesn't care that narcos are killing his people because he's siding with the narcos. I've already mentioned before how most Leftist parties in Latin America receive a lot of their funding from cartels, even if by proxy. Cuba makes a lot of money from drug-smuggling and they give money to political parties that support them. If Trump wants to end the cartels, he would have to be involved in the actual politics of the region, and whether we like it or not, Lopez Obrador was rightfully elected.
Narcos don't go around killing random people, that's a bit of a misconception. The MS-13 do, but the cartels want to be left alone. Their most common victims are journalists who investigate them, anyone who tries to approach to them for whatever reason, or politicians who want to get rid of them. Any alleged innocent bystander who gets killed by them is often an accident or someone related to the people I just mentioned. Mexicans just try to stay away from them as much as possible and that keeps them somehow safe.I'm willing to bet this actually was a fuck-up by the cartels. My understanding is that most of them avoid deliberately killing Americans (especially in insane, high-profile slaughters like this) because they don't want heat from the US government. Robberies and kidnapping, sure, but mass murder drawing international media attention? Unlikely.