I found out this since very long time ago… I found a behaviour pattern that if a person keeping complaining he is a working draining schedule with low wage or boss bullying, that person is going to treating people awfully bad or ranting or starting bullying, especially towards people who don't have a job or antiwork. And they think they have rights to treating (especially antiwork) people as sh*t.
And the target could be their friends, children(this is most often), and even the person who having a romantic relationship with themselves.
This turned to be a common warning signal for me.
If your job is going to make you to treating people/children like this, that is a devil deal between you and your boss, we don't need tolerance such kind of behaviour, no matter how common they are. Just leave that person and tell him:” I know what you are doing, and I don't want to be the external cost of you and your boss while you 2 are earning money.”