There was no reason to kill Fredo. There was a reason to kill Tessio and there was a reason to kill Carlo. The reason is because Tessio was a powerful capo who was setting Mike up to be killed. Carlo was a punk sore at the world who helped plan Sonny's Murder. Now Fredo was not a violent guy I'm sure he had bodyguards working for him but he wasn't a cold blooded killer. I still dont believe he wanted Mike dead. some people believe that fredo's plan was always to have Mike killed but it just doesn't seem like that since Fredo had a good heart. There is no doubt he was pissed about being passed over but he never would hurt one of his brothers. I think at that point Mike became a full fledged psychopath and his paranoia took over