yea that sounds about right, there's spells to become anything here, so of course someone made a 'spell to become a witch'.
gps does make a point, it's just a label people give themselves. but you too are correct as i know many people who just use the fact they're a witch as a way to lord over others or get their way. back in high school i became friends [because she told me we were friends] with a witch who specialized in curses. i saw her curse many people for arbitrary reasons [like they took the last blueberry muffin or something] and she would say rather loudly how great she was at cursing people. it's not what being a witch is about, but some people see it as bubbling cauldrons, broomsticks, and casting on other people.
you don't need a spell to become a witch, it is a way of life, it is a religion, it is many things that cannot be created simply by chanting a few words, then going about your day. yes, you can do a ritual to dedicate yourself to your patron deities, but after the ceremony, you don't flop down on the couch and pretend like nothing happened, which is what this spell sounds like to me.