We know Spire is looking for his race, and is suspected to be the last one. Now go watch the begining cutscene of the game. The title one. Go on, Get your DS and do it. I'll wait. In particular, watch Spire's bit, where he rolls down the hill and smashses a few rocks. Note how he goes out of the way to smash them, and how the camera focuses on his face.
Here's what I think happened. Spire's crystals are possibly a deformity amongst his race, assuming the other rocks in the video are other Diamonts. He was shunned for this, and was reduced to hanging out with a bunch of similar rejects, taking drugs all the time, permanently off his head. Then some kind of civil war happened, and he went nuts. He went so over the top, in fact, that between the war and his murder, the population was quite severly reduced. He may have got ahold of some atomic weapons or similar and started pressing buttons, or, more plausibly, there never were many Diamonts in the first place - how does a rock race breed, exactly? He pried the Magmaul from someone's cold, dead hands and assumed someone had given it to him when the drugs wore off.
Therefore, Spire's intro movie depicts him killing the last few Diamonts whilst depressed, drugged up and crazy, and he doesn't remember it.
- Apparently Spire's backstory was a reference to a cancelled game made by the same studio, so I doubt they put that much thought into it.
- It was a game called "Raven Blade". During a tech demo, a Diamont-like creature is seen tossing a lava ball, possibly inspiration for the Magmaul. Spire's backstory is referencing the fact that Retro Studios (who apparently designed Hunters) dropped the game to work on Metroid Prime, and he is the last evidence of "Raven Blade" left.
Several facts point to this. For example, Sylux slowly being positioned as The Rival to Samus with hints in Prime 3 and Federation Force, leading up to Prime 4. Back in the day, each hunter even had their own trophy in Super Smash Bros. Brawl. It seems the game is being set up to make quite the mark in the story.
Gorea is already an established psychic, able to send telepathic messages across galaxies. When he came upon the Alimbics, a race of psychics, he began to kill and absorb them to become more powerful, absorbing weapons in addition to this. Throughout this entire ordeal, the Alimbics had something that could kill Gorea: the Omega Cannon. However, instead of using this awesome weapon, the Alimbics all opt to kill themselves in order to seal Gorea. They even seal the Omega Cannon in the same inter-dimensional prison. It's then revealed later than Gorea is able to draw more power from the Seal Sphere, the very thing he was sealed with. So here's what I think happened:
Gorea realized the threat of the Omega Cannon and sought to stop it any way he could. He did this by tricking/minraping the Alimbics into a state of severe hopelessness and convinced them that the best way to get rid of him would be the "kill ourselves to seal you" plan described above. So the Alimbics transfer their psychic essence to the Seal Sphere and place him in a space between dimensions. He also convinces them that the Omega Cannon is far too dangerous to use. At this point Gorea has gotten all the Alimbics into a small area that he can eat at his leisure, and time to study the Omega Cannon. He even had a back-up plan: send his extragalactic psychic message to get someone to release him. It would have worked, too, if he hadn't inadvertently called Samus.
