Actually what she said was symbolic... and/or foreshadowing.. She may not be evil. (I personally don't think she is bad at all).. But, I think all the viewers get the idea that Dungeons and Dragons foreshadows the show. (I HOPE THEY SAW THAT CONNECTION OR ALL HOPE IS LOST!!)
According to the game: The Demogorgon appears as a somewhat humanoid form. Two mandrill heads sprout from his twin snake-like necks, and his arms end in long tentacles. His two heads have individual minds!!
One of Demogorgon's best-kept secrets, even from his cultists and minions, is that his two personas strive to dominate!!! and even kill each other!!!, but are unable to because they are aspects of one another.
He first showed up in her subconscious mind.... not in the Upside Down....... By touching him she opened a gate...... So, in a sense she could be "a part of the monster"..... But the good side, and the only way to destroy it, as we all saw in the end.... She defeated him.... She seemed to know where Will was at all times on the upside down..... How would she know that unless she was connected to the monster??? Also, why when killing all the soldiers did the monster let Dr. Brenner (aka. Papa) live? They are dual personalities of the same construct.... Hope that helps.