Kakashi promised to protect Rin. He failed. Obito never want to see him again and the two part ways. A few years later Kakashi meets Obito in college. He is now a feared gang-leader, skilled manipulator and unconvicted murderer consumed with hate. He is determined to make Kakashi pay, while Kakashi desperately tries to pull Obito out of his own darkness. ObiKaka

Just because you don't like what someone had turned to, you can't separate the past form the current. The past can't be erased. The present can't be denied. In the end, you have to accept them both, or not at all. 'To protect Obito of the past, I will kill the current Obito.' That was what he told himself. It wasn't that easy. Dark AU.