"Anti" in the Greek does not mean "against".
It means "in place of",
such as when an ambassador goes to another nation in place of his king or other leader.
In the case of the Antichrist,
he claims he is the vicar of Christ,
or his representative.
He is of course lying,
as much as a traitorous ambassador would be to give false representations of what the King really said,
or as much as one giving false representations to the King of what the foreign nation really said.