In D&D, commitment to Balance involves a belief that the ultimate triumph of good over evil would in fact be horrible. This thesis is advanced explicitly in several places, most notably in Pages of Pain where Troy Denning has the Lady claim outright that the triumph of good would be worse than the triumph of evil, and Troy Denning had a massive influence over D&D fluff over the years. This sentiment is not unique to D&D, the exact same argument is advanced in the conclusion of the Wheel of Time series. This is the idea that both evil and good (and in D&D chaos and law) must continue to exist in roughly equal amounts and any progress made by one side must be halted so as to prevent it snowballing into triumph.

Very few people are actually directly behind this philosophy, since it is counter-intuitive, anti-emotional, and extremely callous overall. This is why the Lady of Pain - who is explicitly disconnected from everything and has no true identity of her own - serves as a highly functional champion of balance and why D&D has portrayed the champions of balance as either inscrutable (Rilmani) or inherent (Aeons). Even a large number of true neutral individuals aren't especially pro-balance, they're siding explicitly with some cause or force that lacks a moral stance - like nature.

The anti-balance group is literally everyone else, because they all have an agenda that ends in the triumph of some specific ideology, for good or ill. However, these groups aren't specifically anti-balance and while most of them find the pro-balance types annoying they have other priorities from ideologies that actively oppose their own all of the time rather than only at certain intervals (and in most fantasy narratives evil is on the verge of triumph, so the pro-balance forces tend to stand with the good guys or at least stand aside).

A dedicated anti-balance stance means trying to break the universe, meaning fighting for an outcome that leads not to the triumph of some ideological viewpoint, but to a total restructuring of the paradigm, possibly down to the point of rewriting physical constants. Given that this usually involves killing pretty much everything alive in the universe as is, the beings behind such things tend to either be absurdly nihilistic or from outside reality as it is understood. In D&D Ilsenine, the god of Illithids, pursued this goal as did various Far Realm entities.