We get this story almost exclusively from Matt's perspective, and he's a guy who is under a lot of pressure. So it makes sense that this book has a lot of tension in it. Matt is often anxious and worried, and his worry comes out in the novel's overall tone. We see it most in moments like this:
He wasn't a clone! He couldn't be! Somehow, somewhere a mistake had been made. [...] Was he going to end up strapped to a bed, screaming until he ran out of air? (13.1)
You can just feel how stressed and upset Matt is in this moment. We're stressed just reading it, and we don't even live in Opium. He's made a huge discovery about his own identity, but he doesn't quite understand it, and he worries over his unknown fate, because his fate is a mystery. Matt doesn't know yet that he's destined to donate his organs to El Patrón.
This mystery only adds to the tension. Matt often doesn't know what's going on, so he's forced to play detective and put the clues together himself. We, too, have to figure things out for ourselves, as we tag along on Matt's adventures. Certain things are revealed to us, and certain things are kept secret. Take, for example, this moment from the opening chapter:
"Don't fix that one," said Lisa, hastily catching his arm. "It's a Matteo Alacrán. They're always left intact."
Have I done you a favor? thought Eduardo as he watched the baby turn its head toward the bustling nurses in their starched white uniforms. Will you thank me for it later? (1.18-19)
Who's Matteo Alacrán? And why will the baby later thank Eduardo for not touching it? We're left to figure that one out for ourselves.