Yeah especially if no knew he was the one doing the killing. If they saw some dead Uchihas and then they saw Itachi they would never even think that Itachi was the one who killed them, in fact they would probably feel safe because he is there. Just look at Sasuke he saw Itachi standing over his dead parents and the thought never even occurred to him that Itachi was the one who killed them.

As for Itachi and Madara killing the clan, there is the element of surprise and there is the fact that it was at night and no one would suspect Itachi. Plus Itachi had MS and none of the other Uchihas would have been able to counter that all he had to do was look them in the eye and its over, beyond that I highly doubt Itachi and Madara took on the Uchihas en mass because then, even as skilled as they are, they would have been outmatched. It would be more likely that they quietly eliminated one small group at a time or probably just moved from house to house. It has nothing to do with weakness.