While not inherently evil, those that employ the art of darkness magic are more often that not at the risk of becoming corrupt by its usage. Long-term use can alter one's personality and push them towards high levels of sadism. Brutal rage and even insanity are not uncommon traits found among users of dark magic. This is attributed to the unnatural adoptment of dark magic by humanity, leading to it's overly symbolic nature. Occult madness is a separate phenomena to Depravity, though it often leads to it.
It symbolizes destruction and thrives off of the caster's desire to destroy. Magi often focus this on a particular, commendable goal, to avoid the destructive tendencies being used for evil- such as only murdering the enemy. Nonetheless, even with certain goals in mind, the effects of the magic can sway a Magi's mindset towards other less respectable objectives, and genuine goodness becomes close to impossible. A pacifist is simply incapable of wielding dark powers.
This magic's murderous urges are incredibly difficult to hold back. The power of destruction brought on by its very nature makes it deal as much damage as possible, and if a darkness Magi enters battle, they must typically do so with the mindset that someone will die due to the lack of restraint they are able to exercise.
Holy Magic, which embodies and encourages the desire to protect, is considered the opposite to darkness magic. The two mana types naturally do not mix well as a result. It is not possible for a human to wield both Darkness and Holy Magic unless they somehow possess Divine Magic.
The Yokai are able to use the magic without it altering their state of mind. Though since their race is born to devour their kin for power, this ability offers them no great benefit it would have for the less intrinsically damaging races. They are natural masters of the art. The spread of dark magic was largely a result of powerful Yokai of Kaor level or above teaching their human servants.