Give us a few sentences so that we understand the context and make sure that one of those sentences is "Instead of visitors, they were thieves".
"Instead of" means "in place of; as a substitute for". "Instead of apples, you sent me oranges."
Your sentence doesn't work well since it doesn't have this meaning.
ems wasn't asking you to rephrase it. She was asking you to use the phrase in a context. I assume that she wanted to know whether you knew its meaning, and I gave a guess.
Why not do what Rover and emsr2d2 have asked first, and give a context which includes the phrase? If you can do that, we'll give you an opinion about whether it works in that context.
Note that rephrasing a sentence is not giving an example of the sentence in context. It's giving a different sentence.
Assuming you mean "instead of visitors, they were thieves", beachboy, I'll try to end your misery.
Yes, I'd accept that usage. I would probably not use "instead". Maybe, "we found that, rather than visitors, they were robbers." There are other ways of saying it. I'm just not sure that the thieves were there instead of (in place of, as a substitute for) visitors. But most native speakers probably wouldn't think twice about this.
We'll see if others have any other point to make.