The Elden beast is described as a vassal of the greater will. It's also the thing that became the Elden Ring.
In the ending cutscene, apart from the lord of frenzy and the age of stars, we see the Elden Ring is still inside Marika, and it's implied your character takes it in order to become Elden Lord. In this sense, I would read it less that you killed the Elden beast and more that you subdued and absorbed it, becoming its next vessel.
As for how shitty some of the endings are, I don't think the Greater Will cares. It didn't care about all the people Marika killed, it only cared when she defied it by shattering the Elden Ring. I suspect that's why all the endings where you take the Elden Ring are basically identical. Nothing has fundamentally changed, you're just the new Marika. You can make the world nice or shitty depending on your choices, but you're still a servant of the Greater Will. The source of your power and the order of the world is still the Elden Ring.
Ranni's ending is different presumably because Ranni doesn't have a body, and thus can't become a vessel for the Elden Ring. The frenzied flame ending is different because Marika's body crumbles away completely, implying that the ring, and thus the concept of order, has actually been destroyed. These endings are different because the whole order of the world is fundamentally changed in some way.
Random aside, it's actually a really weird coincidence that I've been writing about the historical concepts of order and chaos lately, because the game uses the word chaos in the premodern sense. When Shabriri says "let chaos take the world", it doesn't mean "let everything be really confusing and random" or "let there be an absence of stable authority", it means "let the world return to the undifferentiated state before creation". This kind of stuff is actually pretty common with the souls series, and why it's really fun to discuss them with academics, because whoever writes and translates them are a bunch of nerds. It's why I never feel bad about reading too much into them.
At one point years ago I recall that Miyazaki had a canon worked out across the Souls titles but even then, much is intentionally left up to personal interpretation. There’s just no way that with multiple endings people would ever universally accept anything as definitive.
Here it is, in a post with the vg247 link to an interview-
Dark Souls 3: Miyazaki explains the difference between "difficult" and "unreasonable"
The Dark Souls creator talks storyline, challenge, character builds and more.
www.vg247.com