Dark types generally use dirty, underhanded tactics to fight, as evidenced by some of their moves (Sucker Punch, Pursuit, Beat Up, etc.). Evil beings are more likely to cheat or manipulate to win than, say, Fighting types, which signify respectable fighting. Game Freak may have wanted a sort of category to put cruel, sneaky, or devilish Pokemon in, as they just wouldn't fit elsewhere. The dark is often associated with evil. This may be because quite a few humans fear the dark, as we primarily rely on sight. Without this sight, we feel more susceptible to danger.
Even if not all Dark types are inherently evil, their STAB moves are immoral at best. Things like Crunch and the aforementioned moves are quite dirty tactics to use.
It's possible a translation error was made, and that's why the English type is not called the Evil type.
Hope I helped!