"Evil" isn't an anthropomorphised "thing" or "other", just as "good" is not either. Neither has a "motive" as if it was an independent entity.
"Evil" and "good" are simply labels we put on certain behaviors and attitudes, and what we consider "evil" and "good" changes over time.
To think of them as coming from anywhere but ourselves is to simultaneously refuse to take credit for the good we do and to pretend we aren't fully responsible for the evil we do.