The main reason that so many different groups were shown in the series is that RJ was trying to make his world look realistic. If everyone was selfless and noble and willing to sacrafice for the greater good how interesting a book would that be. Hell the series would probably have ended by Book 2 as the combined nations armies and the Seanchan advance guard marched on the blight.

No group except possibly the Shadow are 100% either good or bad. Rand has made mistakes. The Aiel are kick ass warriors but do they ever hold a grudge against the people of Cairhien, most of whome were not consulted by their king before he cut down the tree and quite a few were not even alive at the time.

The Tairain High Lords are arrogant tyrants but their land is orderly if oppressed. Andor is showed mostly in a good light but they are willing to destroy an entire generation to attack a borderland army that has been keeping the world safe for years from the Shadowspawn just because they did not ask for permission to go south. The list could go on.

The Aes Sedai are high handed, think they are the most important group in the world, plot and scheme but consider this. They have stopped wars in the world, they have kept alive knowledge of the Shadow. One of them saved the world by finding the dragon reborn before the shadow. They are a centre of learning and while their methods can leave a lot to be desired the average Aes Sedai's heart is in the right place.