Even thought Black Magic is only illegal because people can't be trusted with it, does not make it any less evil. If you have a knowing willingness to wield a power which is against the law, that you purposely use despite it being illegal, it is 'bad/evil' according to society. Most things that are illegal in the real word are illegal because people can't be trusted. Some kinds of murder are arguably justifiable, HOWEVER . ALL murder is illegal because people can't be trusted if 'some murder' was legal. Same goes for black magic in FFXIV. It's not inherently evil, but it's usually (though not always) used for destruction therefore it's banned.