Hang on here, kinda late to the party, but can I get some sources on all that? Because I don't remember almost any of that, and a quick google of Aldia's dialogue doesn't really support it either, and neither does the information we get from DS3 and its DLC. For one thing, Aldia makes a very explicit statement about it being Gwyn's actions that damned humanity to the Curse of the Undead, and the terrible lies behind it all, and he is outright explicit about it being the Lord of Light (Gwyn) whose actions damned mankind to mortality (a "fleeting form) with only the illusion of life.
I definitely agree that the Linking of the Fire isn't the First Sin, as Aldia clearly states that the whole ritual is the work of a true monarch (rather than, say, a sinner), but the Linking of the Fire wasn't what "banished Dark, and all that stemmed from humanity." That, as shown in the Ringed City, was the seal of fire that Gwyn and the gods placed upon humanity and their works, as explicitly stated in multiple item descriptions (exempli gratia, the Ringed Knight Armor). If anything, the information revealed in the Ringed City provides plenty of strong evidence toward the First Sin being the betrayal of mankind, including:
The information that humanity directly assisted in the war against the Everlasting Dragons, turning the seal from an act of tyranny into one of betrayal, an explicit sin throughout the series.
The statement (from the Ringed Knight Hood) that the use of Abyss-tainted cloth was to "reveal that which the seal of fire has occluded, a subtle defiance of the gods' wishes." This links back with Aldia's lines, "A construction, a facade, and yet... A world full of warmth and resplendence." That the gods would try to specifically hide the Dark, and force humanity to only see the light and heat of Fire seems to link these together rather strongly.
The timing of events between cycles lines up better this way. As Aldia says, "Shackled by falsehoods, they yearn for love, unaware of its grand illusion. Until, the curse touches their flesh." The curse of Undeath that shatters the illusion doesn't sweep over mankind when someone Links the Fire, it returns when the Fire starts to fade, a direct inversion of such a premise, and entirely unconnected to the events of Chaos.
As Aldia takes care to mention, "Peace grants men the illusion of life," and, "All men trust fully the illusion of life." Meanwhile, as each and every item with the words, "Ringed Knight" states, "The [armor/arms] of early men were forged in the Abyss, and betray a smidgen of life. For this reason the gods cast a seal of fire upon [these], and those who possessed them." Emphasis here is mine, but important: these items have life in them, real life, and the gods sealed them away, along with humanity. And, as Aldia states, the "life" humanity since has experienced is fake, with the deception only being revealed once the Curse starts to take over.
Aldia thrice uses the word "yoke," and in a very specific way. First, he uses it to describe the illusion that chains humanity, and that its bondage is "as true as the Dark that churns within men." He then asks if we are "intent on shattering the yoke, spoiling this wonderful falsehood?" Finally, he states, "I sought to shed the yoke of fate, but failed." Here, fate doesn't need to refer explicitly to foretold events, some inevitable future, or a grand destiny, but can also be used to refer to things which have already happened, a state you have already found yourself in. With this secondary definition in mind, and all information from the Ringed City, the meaning shifts to his attempt to be rid of the seal of fire rather than being rid of mortality or undeath (and, as we can plainly see, he is quite fully consumed by fire).
So evidence seems to point to the gods' betrayal of humanity to be the First Sin, especially as this happened just subsequent to the defeat of the dragons and, therefore, prior to both the Linking of Fire and the Chaos Flame. Meanwhile, the evidence to the Chaos Flame sin is... that Aldia experimented with Fire and looks jacked up with tree roots. But the Bed of Chaos isn't the only thing with ties back to tree roots and plant matter. For one thing, the giants were pretty much animate trees, and we know for a fact that Aldia was tinkering with those, so it seems a simpler explanation that the giants might have more to do with that.
Further, we have some (minor, but extant) evidence against Aldia being tied to Chaos by the fact he isn't dealt extra damage by the Black Knight weapons, weapons which (as both DS1 and DS3 show) deals extra damage to Chaos. Now, I know that there are other demons in DS2 and that they don't take any extra damage from the Black Knight weapons, but they clearly aren't the same sort of demons as those birthed by Chaos. The Covetous Demon was once a man, then transformed into a demon (and beings transformed into demonic entities, like the Demonic Statues, Bed of Chaos, and Ceaseless Discharge, aren't dealt extra damage by the weapons). The Demon of Song was associated with water, which rather clearly seperates it from the fires of Chaos. The Smelter Demon is explicitly stated to be a mass of iron given life, closer to a golem than a true demon, and (like the Demonic Statues) wouldn't be dealt the extra damage. The only enemies we face in the Old Chaos are the Charred Loyce Knights, not demons. Meanwhile, the explicitly Chaos-born demons of DS3 do take extra damage against the Black Knight weapons, seeming to indicate that this property never went away, only that nothing in DS2 was a true Chaos demon (Note, I do consider this the weakest point of evidence, being rooted in game mechanics from the black sheep of the series, but evidence is evidence).
Finally, Chaos seems way too insignificant for the much-hyped First Sin to be, "Witch Izzy fucked up, demons popped out, and property values went through the fuckin' bedrock." I mean, Chaos is pretty self-contained and well understood, with limited and localized effects, not some world-defining secret. Maybe in Demon's Souls, but not really in Dark Souls. In Dark Souls, the worst you can say about demons is that they're trying to kill you and they're communists. The former is true of damn near everything in the games, and the latter seems much less of an offence when you compare Anor Londo to, say, Blighttown. But locking humanity away from the truth of their souls, and maintaining a lie that is only revealed when reality itself starts to break down? Now that sounds like something worthy of such a dark accolade.
Also, before anyone brings her up, the Lost Sinner is the Lost Sinner, not the First Sinner. We've got plenty of sinners in Dark Souls, they have their own church and everything. It'd be easier to say that Velka was the First sinner at this point.
Of course, if you have any evidence in the other direction, I am more than willing to hear it out; it wouldn't be the first time I've had my understanding of Dark Souls lore shaken or torn down.
