they never have any proof of itachi killing anyone, for example: him actually killing someone. The only evidence that he would be actually evil and not a great actor would be that he said "kill them, they need to die immediately" to kisame when kakashi spoke of akatsuki. If itachi really killed his clan, there would be no reason to keep sasuke alive. I think kishimoto made the flashbacks that way to later on add something different, possibly a good itachi. Cuz if he wanted more dramatic effect, sasuke could have walked in right when Itachi sliced the parents, while his mom screamed "SASUKE, RUN!" or something like that.