tl;dr– The term "you people" can reference an implicitly-specified nature of the listener apart from the speaker. This term can appear in socially-divisive speech as a mode of addressing a listener socially-divided from the speaker without the speaker having to explicitly state the nature of the social-division.
Background: The term "you" and further qualifications to it.
The term "you" references the listener. In the absence of further qualification, it's often taken to reference the listener as a person.
However, further qualifications can be added. Qualifications can serve two purposes:
Qualify a sub-group of listeners.
For example, if a manager is addressing a large group of employees, they might say
You engineers design the new system.
to address the listeners who're engineers, rather than others who might be listening.
Qualify the nature of the listener to be addressed.
For example, if Alice is speaking to Bob, if Alice wants to address Bob as a member of some group rather than as a person, Alice might say stuff like:
You men [...]
You people who like video-games [...]
You Americans [...]
You who think it's funny when kittens bat at yarn [...]
In those examples, Alice is addressing some nature of Bob's rather than identifying Bob from other listeners.
Alternatively, we can say that the two different usages above are actually the same thing: in both cases, the speaker is addressing some nature of the listeners, filtering out listeners who lack that nature. For example,
You engineers go design the new system.
is speaking to the nature of the listeners as engineers, presumably filtering those who lack such a nature as they have no such identity to hear it.
The term "you people" references the listener with further, implicit qualifications.
Literally, "you people" speaks to the nature of listeners as "people".
One idiomatic usage of this is basically a variant of "you guys", e.g.
Oh, you people are so sweet! Thank you so much!
Oh, you guys are so sweet! Thank you so much!
, where the speaker is basically thinking of the listeners as a group apart from themself, and the qualifiers "people" or "guys" can serve to stress that distinction.
Note: In modern American-English, "guys" would tend to be preferred over "people" in most cases of the above.
In the above case, the speaker refers to others as "people" or "guys" without explicitly qualifying what they mean by that. If we were to show the hidden implicit qualifications, then it might be something like
Oh, you people [who gave me this wonderful gift] are so sweet [for having shown me affection or similar appreciation through having given me this wonderful gift]! [I] Thank you so much [for the wonderful gift that you gave me]!
Oh, you guys [who gave me this wonderful gift] are so sweet [for having shown me affection or similar appreciation through having given me this wonderful gift]! [I] Thank you so much [for the wonderful gift gave me]!
, though generally it's not necessary to be so verbose because listeners can infer the implicit wording.
Point being, the term "you people" references the listener with further, implicit qualifications.
About negative cultural associations with "you people".
As explained above, the term "you people" refers to the listener with some implicit-qualifications.
This term can be especially useful to speakers who'd like to stress an implicit-qualification that they rather not state explicitly. In bitter, adversarial exchanges, it might convey an implicit-qualification of the listener as they differ from the speaker.
Often such implicit-qualifications can involve race, social-class, political-affiliation, religion, or other social-framework with problematic divisiveness.
In short, the term "you people" can reference an implicitly-specified nature of the listener apart from the speaker. Because this term can be useful in addressing a listener in a socially-adversarial context, some have associated the term to socially-divisive speech.