Honestly, if he actually is an Uchiha (and he thinks he is, at the very least) why the hell would he want to avenge the man that KILLED THEIR CLAN!? Seriously, is he grateful that there are like, three left? Why would he possibly seek to avenge Itachi? How would killing the killer's killer (Sasuke) bring honor back to the Uchiha? Is Shin just an idiot?