The assassination of the assassins makes no sense.
It's unrealistic to think that anyone could take out 2 hired killers by cutting their throats. Even a surprise attack by the Family traitor would only kill one.
I agree Woltz. It is highly improbable that two hitmen are going to expose themselves to being murdered from which we infer that they must have been taken completely by surprise from behind. Also, why use a knife (and invite all the variables that can accrue to its use) when it's obviously a situation in which one uses a pistol with a silencer.
Also, remember the line: "I don't recognize them. They must be out of New York." Ridiculous.
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