{What have we to do with thee?} (	i h(886d)in kai soi?) The
same idiom in  Mt 8:29 . Ethical dative. Nothing in common
between the demon and Jesus. Note "we." The man speaks for the
demon and himself, double personality. The recognition of Jesus
by the demons may surprise us since the rabbis (the
ecclesiastics) failed to do so. They call Jesus "The Holy One of
God" (ho hagios tou theou). Hence the demon feared that Jesus
was come to destroy him and the man in his power. In  Mt 8:29 
the demon calls Jesus "Son of God." Later the disciples will call
Jesus "The Holy One of God" ( Joh 6:69 ). The demon cried out
aloud (anekraxen, late first aorist form, anekragen, common
second aorist) so that all heard the strange testimony to Jesus.
The man says "I know" (oida), correct text, some manuscripts
"we know" (oidamen), including the demon.