Think about it--it makes no sense to leave someone alive just because they're too weak. If their whole goal is murder, then they'll take that weakness and kill you without a second thought, just so they won't have to waste any time on you--they won't ever have to think about you again, or deal with you coming back for round two, and possibly being strong enough to matter that time. They'd throw you away like trash just so the job won't go undone. Not killing you means they either can't, or they won't; they're not finished with you yet, either way.
[All this talking is making his throat hurt all over again, but Sasuke needs some way to make sense out of all this. He needs a clearer, outside perspective, one that can grasp logic beyond the emotional trauma the whole situation carries.]
I don't know what happened in Konoha, or why they'd make him kill your clan--but I do know that he doesn't like death for death's sake. Itachi doesn't kill people unless he has a good reason. He doesn't even fight unless the situation demands it. But you and that final fight he was waiting for..... I don't think you know what kind of suffering he struggled through to make sure it happened. He should have died long before you managed to reach him; he held on only so you could be the one to strike the final blow.