Well, honestly I think this is a lot more complicated other than "Don't be a dick", some really nice people can do horrible things with good intentions. Rather than focus on words look more into actions.
They can be really nice, ask you to do them a favour in a pleasant tone of voice but in the end they may just be using you. Flags are obvious signs that someone has bad intentions but the worst type of people you will encounter are those that hide it better.
I disagree with some of these red flags some people have suggested, its all about context and demeanor.
Such as; "lack of communication" an example I would use would be; two people in a room and you're both strangers, you have to talk about something you know a lot about but they don't. Generally most of the time you would lead the conversation because they know either nothing on the topic or they are too shy.
Or lack of topics to talk about (sorry Kat your ones stood out the most to me because I'm both of those. #Tilted whatever) that could just be lack of confidence.
Just because they lack those qualities shouldn't raise red flags, some people need a bit more time and effort but you can find them to be loyal and honest friends.
Here comes the cringy bit, I believe when you realize that each person you pass every day has an infinitely complex life compared to your own because of millions of different variables, that no one is the same but we all have baseline similarities.
You would never just turn away anyone because of a red flag when they have green ones, no one can conform to the ideals of red and green, everything is grey. (Unless they state that they want to kill children, thats a red and you should call the police)
Another example (sorry Draugvan) is you can have a friend that polite to public sector works and having a fair enough attitude and they gossip. Why? because they're honest with what they think. If you think they deserve to be red flagged on the way they speak if they use words like gucci or lit then you aren't really much better than the ones that deserve to be ignored.
It's like a man from England saying he can never be friends with a guy from Scotland because he doesn't like the vocabulary of Scottish words like 'wane', 'aye', 'auld'. Which mean 'one', 'yes' and 'old'.
TLDR: People do bad things, but they also do good things.
A red/green are people that are able to make really horrible dark jokes about themselves. (It's about context and demeanor)
A big red are people that take something and blow it out of proportion.
If this is for depth of character I suggest you make them grey. The best villains are the ones you understand their viewpoints from.