Why did Nero intercept Spock in the first place when Spock was saving Romulus. It was never explained, all Spock said was that "I was intercepted by Nero. My ship was no match for his..." Nero couldn't have had any reason to kill Spock if Spock was the one who was saving his home planet. Only afterwards were they sucked inside the black hole.
The prequel comic series Star Trek: Countdown answers this.
By the time Spock was intercepted by Nero, Romulus was already destroyed by the nova. Spock was just trying to prevent other casualties (the entire Federation was threatened).
Nero was angry because Vulcan delayed in sharing Red Matter research and Spock failed to create the technology on time.