A valid point. Though it would probably be odd that Sasuke would suddenly become obsessed with protecting the leaf when before he never seemed to care.
First, he doesn't need to be OBSESSED, not more than any other loyal shinobi. "Protect Konoha" become his first priority, not his ONLY priority.
Second, he just killed two of the biggest EVER threat to Konoha. He might as well state he went missing nin to kill people Konoha couldn't touch with official\standard\orthodox methods. Hell, he might decide to STAY missing nin for that purpose, giving Konoha plausible deniability.
Itachi was so out-of-touch on how normal people think, he though such a vague command like "protect Konoha" would have resulted only in his brother going back to the village to live like a honourable Konoha shinobi. I bet we can blame this on Danzou and\or Tobirama, too!