The Chozo harvested the Aeion, which is life energy, from the creatures of SR388. The Aeion also ended up inadvertently forming the X Parasites. The X, as we already knew, kill a host and then mimic it. They are life energy, and can mimic lifeforms because that's what they are. Metroids feed on life energy, hence why the Chozo developed the Metroids to wipe out the X. The Metroids, for the most part, wiped out the X, and then after that were hungry, so they turned on the Chozo for their life energy. As the Chozo were going to leave the planet, some Chozo got in an argument, and one Chozo ended up killing others. There's lots of room for interpretation of what happened here, but whatever it is,

it's going to be very interesting. I think there's a few possibilities, the most obvious of which being there's a hostile Chozo faction out there. Another is that maybe the Chozo that killed the others was an X mimicking one? Like an early version of the SA-X, since it used a variant of the suit that was given to Samus.



The Aeion particles in this game were attracted to Samus (granted, so were the health and missiles), in a similar manner to the X. Of course, we all know the Metroids were eliminated by Samus here, so when the expedition to SR388 at the start of Fusion happened, the X ran rampant there. Hence the teaser we saw after the credits very likely showed the Hornoad that Samus shot at the beginning of Fusion, releasing the X that infected her.