Oriol was in fact innocent of the murder. He had burned down the club Axel named after Kika, and hated Axel for sleeping with both his sister and mother (to whom Oriol was incestuously attracted), but he didn’t kill him. On the night Axel was killed, Oriol was having sex with David after shooting up for the first time. David had never provided Oriol with an alibi for the night of the murder because he didn’t want Oriol’s powerful crime lord father to know it was him who first gave his soon-to-be addict son heroin.
Axel’s killer was in fact Anna (Angela Griffin), one of the group of friends who’d left Manchester together for Ibiza in 1996. On the night of his 24th birthday party, Axel told Anna that, disillusioned with the life they led, he’d sold all of the group’s clubs, property and music rights to Oriol Calafat at a bargain price, and had burned all 100 million pesetas on the bonfire that was the centrepiece of the party, so they could all start afresh.
On the night of her wedding to Marcus, Anna had had sex with Axel and when Axel threatened to tell Marcus about it on the night of his party, she held him under the water of the swimming pool until he passed out. Thinking Axel dead, Anna told Marcus what had happened (leaving out the wedding night cheating) and he helped her to put the body in the boot of Oriol’s Alfa Romeo Spider to bury it. (Explaining why bloodstains and Axel’s tooth were discovered in the boot of the car, which Oriol later sold to Cristobal, the son of his father’s rival Pepe Martinez.)
On the journey, Axel regained consciousness and crawled out of the car. Marcus reversed the car over him, but he survived. Anna tried hitting him on the head with a tyre wrench but he didn’t die, so she took the screwdriver he’d used to escape the boot and used it to stab him through the back of the head, finally killing him.
Then, off-screen, Anna and Marcus presumably drove Axel’s body to Almería and buried it in the desert there, on Andreu Calafat’s land, to implicate Oriol, Axel’s most public rival. Marcus then helped Anna to cover up the murder for 20 years, until which point the Spanish statute of limitations on it had expired.