'That' person...
Black magic.
They used the magic with the demonic energy gained from the contract with the demon, negative energies gathered from others' suffering, and deeds are done against nature.
Though not at the level of demonic magic or the skills and abilities that demons use, this kind of magic is the most disgusting magic used by the black mage rakshasas of this world.
The black magic, though sounds like something of any normal fantasy novel, is something much different from the normal magic and low to mid-level magic skills people use.
It's not normal magic but somewhat more powerful than similar-tier magic spells, but in the end, it is still magic.
Sticky, dark, and disgusting magic.
Most mages loth this magic and mages who use it.
They hate it so much that the black mages aren't even considered mages by society.
And naturally, they aren't even considered part of humanity.