“Either is an English pronoun, adjective, conjunction and adverb. As a pronoun or adjective it means one, or the other, of two choices.
you can't say either and give three choices!”

I think you missed the “OR” defining the “to promote some self serving agenda” as separate from the two alternatives preceeding.

“which I can only conclude is to either increase your sense of self importance, promote argument simply for the sake of argument, [b]or to promote some self serving agenda.”[/b]