What part of "La Cosa Nostra" are cats not getting? Omerta?
Carlo was murdered for playing both sides and getting Sonny killed.
Why do you think Fredo would get a pass? Because he's Mike's brother? Fredo is directly responsible for an attempt on Mike's life as well as endangering the lives of his wife and kids.
You don't gotta be a street nikka or even involved in street shyt to grasp the severity of this situation.
Fredo had to die. Whether or not anyone else involved approved of it is irrelevant because letting Fredo slide would have been seen as a weakness not only by Mike's enemies but also his allies...which in turn would also be seen as a weakness by his enemies...