Michael Corleone killing Fredo or Tony Montana killing Manolo

I, truly, never expected Michael to order the death of his own brother.

Tony was an obvious psychopath from the get-go.

Fredo's death for me, easy.

Actually, since it had already been established that Michael would kill members of his own family (especially those family members that help arrange hits on other Corleone family members), upon reflection it shouldn't have been all that surprising that Michael would kill Fredo.