Why do the Mer hate him so much?
I read that Talos is the ghost of some spirit that returns to defeat Alduin to prevent the end of the world on a cycle. Is that true?
According to the game lore, Talos is what keeps Nirn existing. As someone said, whether Talos is good or bad depends on whether you like the fact that Nirn exists or not.
The Thalmor are religious extremists who took over the Summerset Isles in a bloody coup who literally want to destroy the world because they hate physical reality.
While this sounds quite insane (and probably is), you need to keep in mind the Gnostic underpinnings of this sort of belief, which is ever-present in the game lore of TES:
In Gnostic belief, only the perfect, the eternal, the divine is good. All physical reality is a corruption of the perfect divine. Life and mortality are evil. The world in Gnostic lore was not created by the True God, but by something called the Demiurge, a jealous sub-deity that tried to mimic the True God by creating on its own, but created mortals by mistake because the Demiurge wasn't as good as the True God. All forms of physical being are inherently corruption upon the true divine spark that resides within each being. Any sort of physical pleasure or diversion, then, is corrupting, and we should live purely for the purpose of purifying our souls to be reunited with the divine when we can finally escape our mortality.
In the Thalmor belief, Lorkhan was pure evil for creating the world and "corrupting" them by letting them become living creatures. They want to destroy the world so that they can go back to being nothing but pure ephemeral souls. To do this, they have to destroy Talos, who holds the world together.