There is a difference between opposite spheres and corrupted spheres.
Every god but Dagda has an opposite sphere, represented by another god. (Dagda is the opposite of any sphere taken too far.) Before the fall these were probably more complementary than antagonistic.
Dagda/Balance
Lugus/Light - Esus/Trust/Deception
Sirona/Spirit - Aeron/Strength/Rage
Bhall/Fire/Passion - Danalin/Water/Serenity
Nantosuelta/Enchantment - Ceridwen/Dimensional
Nemed/Arawn/Sucellus/Life - Arawn/Death
Junil/Law - Camulos/Peace/War
Oghma/Matamagic/Knowledge/Memory - Mammon/Foresight/Greed
Kilmorph/Earth/Responsibility - Tali/Air/Irresponsibility
Sucellus/Nature/Gradual Change - Mulcarn/Ice/Stasis/Reactionaryism
Amathaon/Fertility - Agares/Hope/Despair (ok, I don't really see how this one works before the fall)
Some spheres are also rather close together, making multiple spheres seem like opposites of some. Sirona could be considered the opposite of essentially any evil god
Each sphere entails a range of closely related aspects. The fallen gods still represent the same sphere but in a fallen, corrupted way, in which the darker aspects are emphasized.
The Angel of Hope became the Angel of False Hope, which leads to Despair.
Aeron's strength to protect the weak became strength for strengths sake, a right which the strong had to use against the weak.
Camulos once represented the peaceful anarchism of everyone getting along without need for laws or external authority. That became a total rejection of external authority, a belief that all views are subjective so one cannot be better than another, and the view that every individual should have complete freedom to do to anyone ese whatever he wants without needing a reason. This changed Harmony to Strife.
Mammon's ability to see the future changed to a need to be in control of the future by controlling physical possessions, political power, etc.
Mulcarn really did not change much, he just became slightly more extreme.
Ceridwen's sphere of positional worth went to the extreme of denying all intrinsic worth.
Esus's trust became the ability to abuse the trust of others.
Archangels often represent a slightly different aspect of the sphere than do their gods, although they are close enough to be of the same alignment. Note that while they are heavily dominated by one sphere their souls do however contain aspects of other spheres giving them freedom to act opposed to their spheres when they so choose, a freedom which gods lack.
Tali is very good-natured and fun-loving, but Leucetious (Bringer of Storms) seems to represent the danger that comes from irresponsibility.
Sabathiel seems to represent the loyalty that comes from love of justice, while Junil too often seems to represent legalism and doing what is right only from fear of consequences.
Basium hates Sucellus, and embodies the life sphere by sacrificing all to continue his war against evil, while Sucellus embodies it by sacrificing himself in a failed attempt to stop Mulcarn and returning to bring healing to the world.
Any sphere taken too far can be destructive. Too much Spirit can lead to pointless self-sacrifice, essentially suicide. Too much Law is a total lack of freedom. Too much Sun is tactless, biting criticism and unbridled nosiness into the private affairs of others.