So the first one isn't so much a headcanon as a rewritten history that ignores how canon actually went, and the second one builds off of the first, but here goes nothing:
1: Spiriah's experimentation on the Skakdi led to the downward spiral of civilization.
Zakaz was responsible for a lot of the heavy manufacturing of transports and large machinery that was used in the upkeep of the MU, and when it descended into anarchy, both Metru-Nui and Xia offered to take up the slack. However, the adjustment process was a slow one, especially with the roaming gangs of Skakdi making it difficult to transfer the requisite items and machinery off of Zakaz to begin with.
Odina and Xia had both been in charge of overseeing and facilitating the economics of the entire universe, so when Xia began to shift more toward manufacturing, a lot of their responsibilities were in turn pushed off onto the Odinians. This eventually erupted in conflict, the war that left Ancient and The Shadowed One as the only surviving Odinians at all.
Without anyone to oversee the economic flow, Mata-Nui was further weakened as imbalances and inefficiencies began to make themselves known. This in turn led to rising tension in Metru-Nui itself, and the Matoran Civil War erupted. Makuta Teridax brought it to a violent, horrifying end, and soon after the Krahli (and then later, the Vahki) were instated.
However, the war was enough to send Mata-Nui into a slow death. While Toa Jovan and his team raced to find the Mask Of Life, the League of Six Kingdoms arose, the Barraki gaining huge followings with their promises of creating a better future. (and they truly meant it, too, though of course there was a great deal of powerlust involved as well)
Makuta Teridax once again brought that to a halt, disbanding the armies, and after Botar had taken the perpetrators, systematically erased all memory of them. Soon after, Jovan's team restored Mata-Nui, and while conditions did not improve after that, neither did they get any worse, until Teridax's plans in Metru-Nui came to fruition and sent the universe into Cataclysm.
2: Roodaka and Sidorak didn't start out evil. Sidorak was a Steltian trader who got offered a job. And then a promotion. And then a few more. And his ego swelled, and his conscience died.
Roodaka was initially an idealist, seeking to better the harsh conditions Xia now endured, and she knew that she would need power to enact change. However, as she gained power and got better at gaining more, her morals were more and more pushed aside, as she justified her actions as being for Xia's sake. At one point, however, she simply forgot, and gave in fully to the new, manipulative persona she'd created for herself.