Pretty much this, Garrosh didn't want to betray anyone, but his guilelessness has been the main cause of the event; he accepted a deadly Mak'gora like Cairne did, and okay, but he also has been tricked and easily manipulated by Magatha, despite he knew her background and her well known opposition against Cairne's rule, foolishily beliving that all her sweet words for him were sincere. This make him if not utterly guilty, more responsible than anyone else about how the whole thing ended.