4/ Toxic people are hypocrites
They talk all nice with honey in their voice in front of people, and the second their back is turned, the fangs are out it’s criticism time. Toxic people have a real need of belittling those around them, so as to always feel more valuable.
If, for example, your partner is toxic, he won’t stand it when your mother says “The neighbor’s son is adorable, and he’s got such a good job”. When most people wouldn’t even think twice about it, he on the other hand will bring it up the very next evening. And say bad things about the neighbor’s son whom he doesn’t even know, or even about your mother who spoke about him.