LogFAQs > #971201831

I don't remember if we know why, but Marika was working to get rid of the Greater Will for a long time. When she cast Godfrey and the Tarnished out of the Lands Between, it was explicitly because she was planning to have them one day return.
My Lord, and thy warriors. I divest each of thee of thy grace. With thine eyes dimmed, ye will be driven from the Lands Between. Ye will wage war in a land afar, where ye will live, and die. Then, after thy death, I will give back what I once claimed. Return to the Lands Between, wage war, and brandish the Elden Ring. Grow strong in the face of death. Warriors of my lord. Lord Godfrey.

The light of grace that guides you to your checkpoints is from her. She's the one who wants you to go around and collect Great Runes. Melina is heavily, heavily implied to be her daughter which means she wants you to burn the tree.

This is absolute supposition, but it's also possible she was involved in Ranni's plot to kill her son.
The assassins that carried out the deeds of the Night of the Black Knives were all women, and rumored to be Numen who had close ties with Marika herself.

Radagon and Marika were separated for some time. It's possible they were once two people that the Greater Will conjoined. It's also possible that like D and his brother, they were one soul split into two bodies. But we know for sure that they weren't always just one person switching forms like you see in the boss fight.
O Radagon, leal hound of the Golden Order. Thou'rt yet to become me. Thou'rt yet to become a god. Let us be shattered, both. Mine other self.

So putting motive aside, Marika's plan is actually fairly straightforward. She hides herself an army by divesting them of grace and banishing them from the Lands Between. She shatters the Elden Ring which causes the Greater Will to abandon the world. She begins giving grace back to the Tarnished, returning them to life and drawing them back. Most of them fail, but she finally finds one that begins to succeed (ie you). Her daughter, the kindling maiden, joins with you and burns the Erdtree. You recover Destined Death so the tree will actually die.

But then there's the hiccup in her plan because you were just the errand boy. Looking at the cutscene for Godfrey, his guiding grace is pointing towards you. You were meant to do the busy work, then Godfrey was meant to murder you and reunite with his wife. That does not work out for him. So instead of she and her husband ruling the Lands Between, free from the burden of the Greater Will, you get to stick your dick in the turkey instead.

Obviously there's some degree of supposition to this, but I think it's the best idea given the lore tidbits I know.