The conversation and battle with Danzou pushed him over the edge. Not only did Danzou 100% confirm the truth about Itachi, but he freaking insulted him as well. Plus he had like 15 sharingans on his arm and one in his eye socket. Thats a maximum of 16 relatives and a minimum of 8, not including itachi. Plus the fact he helped kill Itachi (there is no way in hell Sasuke could beat Itachi. He died solely from disease. Sasuke only drained a bit of his energy during their battle). His reaction may not be logical, but its human. That being said, I think he's become a good villain, but still believe Naruto is still top dog.....frog.
If you're half the man I am today, you're twice the man you were yesterday.