Confessions of a cartel hitman. I wouldn't say it's an amazing book but it's def worth reading, it gives you a good insight into the cartels but I felt it could have been better. It's cheap as fuck amazon or ebay and I'd recommend reading it if you have an interest in the cartels.
https://www.amazon.co.uk/El-Sicario-Confessions-Cartel-Hit/dp/0099559951
It often starts relatively innocent, being recruited as a look-out, messenger or mule, but once you are in, there is no way back. You're also not in a position to disobey orders, so if it is decided that instead of having to run some errands you have to kill a man, you kill that man (or be killed yourself). The more you climb the cartel's "career ladder", the more money, status and influence you get, but also the more entangled you become with the cartel.
I'm sure the cartels have plenty of psychopaths within their ranks that revel in gory shit, but many will resort to alcohol and drugs to numb their senses before going to "work".
I highly recommend Molly Molloy and Charles Bowden's <em>El Sicario: Confessions of a Cartel Hit Man</em> if you want to gain a bit more insight in the live of a sicario and the inner workings of a drug cartel. Reads like a novel, but is jam-packed with background info.