"User123 is saying some outlandish stuff, I think they're hobbernocking us. As in, they're pretending to support our opinion just to make our opinion look bad."
(hobbernocking just a fake word for example)
I could only imagine this word being used in a negative connotation, unless someone were using it to positively affect an opinion.
I believe "demonize" could be a proper word here, but it doesn't really have the concept of "going behind enemy lines" to make something look bad. At least I've never seen it used that way.
Compound words and phrases are both acceptable answers.