itach was already in the "orginization" when he killed his clan, and orochimaru had already left ( if i understand the timing correctly). i think he might of killed his entire clan to keep oro from gettin the sharingan. maybe after oro found out itachi was stronger then him he wanted the sharingan, and then itachi found out. will a hole clan of urichas it owuld be easy. so he killed them all so oro couldnt get it. he left suseke alive cuz he felt bad he had to do that and wanted sum1 to kill him.