Honestly, I think it was more appropriate for Sasuke to kill Karin than to let Danzou get away. One thing that's been bugging me for just about as long as team hawk's been around... what makes these guys so special that Sasuke will do what he can to protect them, yet simultaneously be so willing to cast away his bonds with team 7?

The answer, of course, is that he only cares about teammates for as long as they bring him closer to his goals. As soon as Karin became an obstacle, it was time for her to go away.