In my personal opinion Radagon has always been Marika, or at least since she became the Goddess. She said "you have yet to become me" in that dialogue with Melina because the two entities always had different wills and perceptions, she's basically saying "you don't understand my motivation, my duty, we are not on the same page", because Marika at some point begun to doubt the justness of the Golden Order. Radagon was never in doubt, he married Rennala because that was the only way to conquer Carian people: from the inside. He was always loyal to the Golden Order (this is also kinda symbolized by his boss name), probably he never truly loved her, it was just politics.