How does the diathesis-stress model account for psychopathology?
You need both a vulnerability and a stressful situation to develop mental illness
Psychopathy is literally illness of the mind, the development of which can be explained using the diathesis-stress model.
The diathesis-stress model states that you have to have both a vulnerability (diathesis) to a particular condition, and an environmental stressor or stressful situation to push you over the edge to develop mental illness.
The vulnerability or susceptibility to a particular condition, such as schizophrenia, could be biological or genetic. For example, if your family has a history of schizophrenia, there's a good chance that you could develop it too, although this genetic diathesis isn't enough on its own.
You also need an emotionally challenging situation, such as a divorce, migration or abuse. This is called the environmental stressor, and is required to activate the diathesis. (Without any diathesis, it is not possible for just a stressor to cause a condition to develop.)