Maybe, this is just my own way of thinking, but, I think that sometimes we can feel driven to know a definite diagnosis, when dealing with a toxic person----because we want to be reassured that it is "them" and not us, at fault.
With a toxic person as well as addicts/alcoholics we are often blamed for the conflict in or the demise of the relationship. If we can be assured that the other has a diagnosis for their behavior, this can be validating for US. We can let go of a lot of the "guilt" and weight of the blame for the problems in the relationship.
Especially, if we are not sold in our own self esteem and personal boundaries----if we have s strong need External Validation----having the diagnosis/label can give us more confidence and a "right" to our own thoughts, feelings, emotions.