Tui and La are Push&Pull, one aspect of Yin+Yang. When Zhao kills Tui, the MOON turns dark, not the world. The sun will still rise in the morning and enlighten the world.

Raava and Vaatu are the primordial forces of good and evil, other aspects of Yin+Yang. They are even stronger spirits than Tui and La. Only higher spirits were shown in ATLA, the foxy helpers and the annoyed monkey being the only examples of lower spirits. Most of the spirits we see in ATLA are powerful enough to manifest in the real world. This does not contradict itself with the small cute spirits of LoK. See it like this: Earth King Kuei is represents a rare powerful spirit while his citizens are the fluffy bunny and lesser dark spirits.