Negatively doesn't mean something is evil, those are two different things. Birth by Sleep introduced the concept of Unversed; negative feelings given form. Negatively includes things like sadness, which is a "dark" emotion, but it's not evil. For something to be evil implies that there is some conscious decision behind it. Darkness is a part of nature, it exists in every heart (aside from the Princesses of Heart and Ven) and it's up to the person to become evil.
Light and darkness are two sides of the same coin. Neither are inherently stronger than the other, but like with darkness, it depends on the person using it.