They were civilans. Most aren't going to understand what Matamoros is like, and thus, believe they were there for drugs or whatever. This is just a morality play by people, to blame someone for a tragedy. They deserved it! It's pretty sicking. Just look at the way they are dressed, and who they were. They are regular family people. Probably not a lot of money. Underemployed.
They aren't going to Moros for a meet, during high alert, even a contact would have said DO NOT come down. You don't just roll into Matamoros and try to find a plug. They aren't even at that level. Moros is CDG, heavy hitters
It's a war zone. There are convoys of armed gunmen on high alert, tactical vests and automatic weapons, it can be like a militarized zone when it's active, which is often. They will empty out into your car, your house, they will send gunmen into a hotel and drag out people, if they even think you are a rival. The exact exact details are unclear, but it was mistaken ID, plus high alert.
Look at actions and facts, not words and innuendo, and what a lot of people THINK about the drug game. They left the victims alive, because they knew they fukked up. They couldn't undo what they did, but sparing their lives eases the pressure.