This is a question that does not quite fit the bill on any particular SE forum but Academia SE comes the closest in my opinion. I will use non-binary they/them pronouns to retain some amount of anonymity.
Some background will be important: I have lived with my current roommate -- a grad student at my university in the US -- for two years, in this time I have come to know them in a way I really wish I hadn't. This person is extremely manipulative, a compulsive liar and used me and my friend for an embarrassingly long amount of time before we realized what was going on. This has not been limited to personal favors but also professional ones where this person would get editing feedback on their extremely long papers with nothing in return, not do the reading and then ask for summaries that they would then use during class discussion without attribution. This is a person who openly flouts page limits, writes in a needlessly convoluted sentences and has shown repeated patterns of not respecting other people's time. Professionally, I think this person is a total hack who uses trauma of marginalized groups to progress their own academic agenda.
Now, I suspect that this person likely has narcissistic personality disorder but I am in no way an expert. They are usually very polite and cordial to other people they do not know very well and generally have a good image in the department -- except that they often do not respect other people's time but people think it as a quirk.
My concern is that this person will soon be assuming a position wherein they will be teaching and advising students (in a different program than mine). I do not know where they draw a line regarding abuse so it's hard for me to judge whether this is something that they might do to their students. Still, is there a way for me to warn someone about this individual?
I am also afraid that if I do make complaint I will be ostracized. I have, with my own eyes observed departments side with abusers because everyone knows them to be "good guys" and I wonder if I should just let things play out and hope other people catch on eventually.
Update: Seeing your abuser succeed in their professional lives without suffering any consequence while your work suffers because of the trauma, that just doesn't seem fair. I maintain that this person is unfit to teach by their history of disregard for rules -- including syllabi, contracts and so forth. In any case, it is perhaps time I sought helped and moved on with my life.