On a personal level I couldn't fathom any circumstance of killing my own brother, ever.
Of course no one wants to think about having to make a decision like that. But if you cross that scenario in the real world and like Fredo, your brother tries to have you killed once(drapes), maybe twice(Fredo maybe telling Roth that he wont live to see the new year). Do you really let him stick around for more attempts? He may succeed!
"This is life and death"
