Remember, the X and Metroids are "natural" enemies. In Fusion, the SA-X was willing to die in order to destroy the Metroid lab and an X core willing to blow up BSL along itself. Later, a SA-X "saved" Samus from the Omega Metroid which in reality was just a self-preservation method.

The Quiet Robe copy then reassumes its red X form and deliberately lets itself be absorbed by Samus, giving her access to Thoha genes and regaining full control of her Metroid powers so she does not endanger her Gunship.

Intriguingly, the X copy of Quiet Robe seemingly helps Samus control her Metroid transformation at the cost of its own life and for seemingly no benefit to the X species, allowing her to escape the disintegration of the planet.

Chronologically speaking, SA-X is the second doppelgänger/clone of Samus Aran, with the first being Dark Samus.