What a great expression!
The one and only time I heard this little word gem was in the 1990 film Metropolitan by Whit Stillman. 
In it, the character Nick Smith is bemoaning the upcoming visit with his father and his new wife. She invited him over to spend some of the Xmas holidays with them and Nick is suspicious of her motives.
“I’m about to go upstate to the domain of a stepmother of untrammelled malevolence, very possibly to be killed… and I get this!” ‘This’ being what Nick calls ‘whining criticism’ of his behaviour.
To give you a taste of some of the ‘whining criticism’ levelled his way…
Jane: You’re completely impossible and out of control with some sort of a drug problem and a fixation on what you consider Rick Von Sloneker’s wickedness . You’re a snob, a sexist, totally obnoxious and tiresome, and lately you’ve gotten just weird. Why should we believe anything you say?
Nick: I am not tiresome. 
As I believe I have mentioned in a previous article, I went to a very preppy law school. I finished writing my Bar Admission Exams and was called to the Bar the year Metropolitan was released. Many of the movie’s characters reminded me of the preppies with whom I studied.
I wasn’t allowed into their inner circle (no surprises there). They wouldn’t have exactly considered me PLU. Besides, to a large extent, I was the ‘token ethnic’ at school.  Hardly a welcome addition to their social group.
(Nick [to Tom]: “There’s something a tiny bit arrogant about people going around feeling sorry for other people they consider ‘less fortunate’… Has it ever occurred to you that you are the less fortunate?”)
Nick Smith is the person I wish I was when I was at law school. Suave, preppy, sophisticated, clever, good-looking, witty, cynical, charming, well-mannered, well-spoken and a member in good standing of the urban haute bourgeoisie. In other words, the kind of person I’d never met before going to law school.
While I myself am not a person of untrammelled malevolence, I’d like to be the kind of guy who can use the expression ‘untrammelled malevolence’ in conversation without coming across as Zero Mostel at a debutante ball.
Luckily, I was never foolhardy enough to try to pass myself off as one of the UC. The entire enterprise would have been doomed from the start. Like the narrators say in those wildlife programs, “Sadly, there could be but one outcome.”
So a tip of the top hat and a clink of the champagne glass to Nick Smith, a young man of untrammelled malevolence when it comes to the titled aristocracy! 
 Re Rick’s fixation: “Rick Von Slonecker is tall, rich, good-looking, stupid, dishonest, conceited, a bully, liar, drunk and thief, an egomaniac, and probably psychotic. In short, highly attractive to women.”
 To see the scene in context, click here.
 To give you an idea of just how preppy my law school was, I… an Italian Jew… was the token ethnic (two birds with one stone!) In a student body of 450, there was one black guy… and he was the preppiest of the bunch!
Nick: The titled aristocracy are the scum of the earth. What really makes me furious is the idea of a whole class of people, mostly Europeans, all looking down on me.
Sally: You always say ‘titled’ aristocrats. What about ‘untitled’ aristocrats?
Nick: Well, I couldn’t very well despise them, could I? That would be self-hatred, which is unhealthy.