I think that these forerunners are corrupt in some way because (semi-SPOILERS) the symbol at the end of the demo video (an ancient evil...) is the glyph of the Didact, who we can assume is the voice we hear as the glyph rotates. The forerunners like blue/purple stuff, and the Didact's original symbol is blue, not red/orange as seen in the demo. Also there are "knights" that are blue Not red, flashed in the ending montage (demo).
Red is associated with rampancy in the halo series, but I could be terribly wrong and it could just represent aggression or an enemy.
Forruners armor, structures, and weaponds utilize "hard light" which is energy turned solid. The semi transparent blue bridges in the games is an example. Forerunner ships, armor, and now I am guessing their weaponds use hard light as well. Forruners tend to make the things mentioned above with more hard light than actual physical material, this explains how the weapons construct them selfs as they are picked up. When a construct such as the enemy AI (cortana mentioned they are AI) is destroyed the hard light no longer keeps the physical portion of the enemy intact, that's why they fall apart.
My sources are z books, wiki's, and my opinion, I am not claiming certainty of this stuff so go away trolls.