The marines, for whatever reason, couldn't be bought off.
That line sums up part of the book for me. The stated purpose of the story is to explain how the assassin El Rey got to be who he is. But it doesn't. It explains how he learned his skills, but aside from a brief, summarized romance, we get very little insight into the man himself. In brief, El Rey, for whatever reason, becomes a hard, cold, amoral killer.
The plot itself is not bad. It keeps moving, with detailed setups for El Rey's crimes, and detail about the types of weapons he uses, if that's your thing. It's unfortunately not mine, so while the plot was fast moving, it didn't carry me along.
A lot of the elements are tried and true thriller material, and the prose is at times a bit heavy-handed. Aside from El Rey, there's not much question who's good and who's bad, and there's a pretty heavy burden of cliches.
If you're looking for an undemanding beach or airplane thriller, pick this up. If you want something with a little more character and psychology to it, keep looking.