Definition:
1. poisonous.
"the dumping of toxic waste"
2. very harmful or unpleasant in a pervasive or insidious way.
"a toxic relationship"

You make a great point. People are not toxic. Behavior can be metaphorically toxic. It can be so detrimental, those around it feel ‘poisoned’. It’s become a trendy buzz term and it is hurtful to say a person is toxic. I think it’s better to focus on someone’s behavior being unhealthy, not the person themselves.