I thought it was obvious that towards the end of season six just before Chris died that he hated Tony, and was full of resentment towards him. All he did was complain about Tony.

Tony also could no longer stand Chris. Tony realized he wasn't going to be the leader he tried grooming him to be. He worried that any day Chris would flip. He was a junkie, and it was seen as a weakness. He had given him a chance to be clean, then Chris was continually teased about his sobriety. He couldn't fit in with the guys anymore. The main reason Tony kills him when he had the opportunity was because deep down he knew it was inevitable Chris would flip. Which is why he felt so relieved after His death.