Sakura did have the intention of killing him, hesitant as she was. Sasuke took advantage of her distraction.She was still unsure of her decision she could have killed Karin but Sasuke didnt want her to join him anyways so he tried to kill her
I'm sure Sasuke knew this and acted accordingly. Or prehaps he just wanted to kill off another bond like he did with Karin.
Karin was "killed", because Sasuke had no more use for her. I think it was something about "if you can become a hostage than you're useless".