Homer + Ovid = Paris
Ulysses + Julius Caesar = Nimrod
Guy de Montfort - Cleopatra = Medusa
Epicurus - Pluto = Dido
Hector + Tisiphone = ?
My answer is
The main idea is to associate every name with its corresponding Circle number in Dante's Inferno (Divine Comedy).
Homer and Ovid both belong to the first Circle, while Paris belongs to the second: $1+1=2$.
Hector is in the first, Tisiphone between the fifth and sixth,
so the result must be
between the sixth and the seventh Circle.
The only creature that satisfies that condition is
the Minotaur, who's the guardian of the seventh Circle.
the title says "diabolical", which is a reference to Inferno.
Note: my previous answer was Pier della Vigna, which is wrong.