on her mind = in her mind?

It means she was actively thinking about retribution.

The phrase "on her mind" is idiomatic. This phrase is used in several songs I can think of (right off the top of my head).

I suppose you could change this to "in her mind", but it wouldn't seem as strong.

Got it. Thank you.
The nuance matters here.

If something is "on somebody's mind" we generally mean that they are preoccupied or distracted by that thing, that they are thinking about that thing much more than they are thinking about other things, or that they cannot stop thinking or worrying about it.