Why is "bono" in the phrase "cui bono fuerit" in the dative/ablative?

Answer 1

In short, the case and usage to which cui bono refers to is usually referred to in English as the "Double Dative".

As the name "double dative" suggests, this Latin construction is made of two nouns in the dative case, followed by some form of the verb to be, e.g. est:

1. An "ethical dative": usually a noun or pronoun referring to a person or people.
2. A "dative of purpose": nouns that include "benefit", "danger", "joy", "emolument", "worry" etc.

The standard translation for the double dative in English is:

1. It is a (dative of purpose) to (ethical dative).

Examples:

1. Rei publicae cladi sunt. (Caesar)
2. They are a ruin to the republic.

The ethical dative can be used with a substantive adjective:

1. Vivit is qui multis usui est. (Seneca)
2. He lives, he who is a help to many [men].

It can also be used with personal pronouns:

1. Tibi curae sum.
2. I am a concern to you. [better put: You worry about me.]

In terms of cui bono, Roman orator Cicero was known to style this phrase rhetorically:

1. To whose advantage?

or

1. Who will benefit?