I watched an episode of ER today and there was a woman character in it who screamed obscenities from her bed to anyone who walked by. Many regarded her as unpleasant and blamed her for her behavior. I saw a woman who had been badly abused – probably as a child and teenager, and she was repeating the things that had been said to her.
No one wakes up one morning and suddenly has a head and a mouth full of ugly words. Those words are recorded in our minds by those closest to us who say the things we end up believing about ourselves. And we will repeat many of the same things to those around us.
The words you, or your characters, speak are the words that run through others’ minds when they are least expecting it.