well the cosmic powers diagram is as written by the perspective of "THE TITANS" this was confirmed at one of the blizzcons story panels so ofc they align arcane as an opposite to their foe "Fel" or as they describe "Disorder" where just like light and shadow they are just as bad as each other
Also any one of the cosmic power "winning" life ends up with uncontrollable growths as see in WoD with the spore mounds, death ofc is bad for obvious reasons but there has to be some death otherwise life becomes unbalanced
the same is said for light and shadow, when light starts winning it brings with it fanaticism killing all non believers as seen in the Mag'har orc race unlock quests Shadow agian ofc is bad things end up being taken over by old gods and such madness insanity.