Does “manada” have sinister overtones in Spanish?
Well, it didn't, at least not until the rape case you mentioned made it to the news.
The men who committed that crime had a chat group named "La Manada" in an obvious reference to the movie The Hangover, where the protagonists called themselves "the Wolfpack" (which was dubbed in Spain as "la Manada" — as it should).
The name "La Manada" was very graphical and catchy, so the media started using it to refer to any group of men involved in a gang rape: la Manada de Manresa, la Manada de Callosa, la Manada de Cambrils...
There are indeed some pejorative uses of manada that predate this one:
- "manada de", functioning as a kind of qualifier: "del metro salió una manada de gente";
- "en manada", functioning as an adverbial expression: "salieron por la puerta de clase en manada"
Both are used to talk pejoratively about people moving in a rather mindlessly way, like a herd: people coming out of a subway during rush hour, kids storming out of the classroom when the bell rings...
However, we should notice that in those cases manada is always part of an expression: manada de ...; en manada. This is different than using just manada as a noun.
With that in mind, I wouldn't say that the word manada by itself has sinister overtones, it just means a herd of animals; but it certainly cannot be used to refer to a group of people anymore, at least not in a light tone like in the movie.
Whether this new meaning will stick or not, time will tell.
PS: Since this word is now used to talk about man-on-woman rape, it is worth noting that women may get a different vibe from the word manada than men do. My answer is necessarily constrained by my perception of the world as a man; it would be really good to get some comments or answers from women as well.