I am wondering if there are any known persons that defected from the Rebellion to the Empire. There seem to be quite some defections to the Rebellion (for examples, see here), but nowhere have I found anything on defections the other way around or even double agents from the Empire.
Maybe the Rebellion was just a very nice place to hang out, but I can imagine there would always be those that regretted joining/being in the Alliance; maybe because they did not see a bright future for the rebellion anymore, maybe because they they did not approve of civilian casualties during missions (I don't even know if this was the case), maybe they just did not like the climate on Hoth.
Now I can imagine that the Empire would not be very forgiving towards rebel defectors, and the prospect of being 'questioned' for information, followed by execution, would probably not appeal to any rebel in doubt. Still, there might have been those that somehow (believed they) negotiated a favorable outcome for their future, in exchange for whatever information about the Rebellion they might possess, that could help the Empire.