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He could also turn him, get some intel and let the cartel deal with him when they find out. The only camera would be an iPhone in that scenario.
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This, which I also assumed was just the setup for the next movie.
Even if he was pissed about getting shot, we are left to speculate that maybe Alejandro needed a cat's paw within the cartel, and he probably thought he was too old (and too well known) to do it himself, so the kid who he knows is broken anyway would be an ideal candidate.
If there is a third film, we will probably see Alejandro exact further revenge, and the kid will be in there somewhere to establish that plot. What happens and how that relationship is defined (fear, loyalty, pragmatism, regret, some mixture) will be up the writers. Hopefully they do a good job, because they are working with one hell of cast.