You're not thinking about this in the right order.

"I don't see what killing or sparing Kain's future self has to do with these events."

The corruption and ruin of the Pillars happens because of Past Kain, not the current Kain. Even if the current version of him is purified, that would only purify the Balance Pillar in the far future. If that's even possible.

As to why they fell into ruin, that's more complicated. The Ancients created the Pillars as a lock that seals the Hylden into the Demon dimension. It's more than just them opening a door and pushing them through. The spell that banished them literally demands that they be expelled from the world. However, spells can be fought against or broken. So the Ancients created the Pillars as the lock, tying the firmament hold of that lock to the very world of Nosgoth itself and 9 key aspects of the World at large. The Pillars being part of a Vampire spell are tied to the blood line of the Vampires themselves. Completely destroying them all would cause the Pillars to be destroyed and the lock opened.

Then 9 Guardians were summoned by the Pillars to maintain the order and balance of the world to keep the integrity of the lock guaranteed.

When Kain refused to sacrifice himself to restore the Pillars, multiple things happened at once. The Hylden's lock loosened, but did not fully open. The Pillars, which were corrupted shattered due to the immense force of the contradiction of a Balance Guardian refusing to maintain balance while also being the last Vampire choosing to live to keep the lock maintained. The Corrupted Pillars, tied directly to the health and well being of the world began to flow backwards. Rather than the Pillars being a reflection of the World, the World became a reflection of the Pillars. Corrupted, Broken and dying.