I think we are all narcissistic to some degree. We are programmed/wired to survive and even though we are capable of truly caring for others, we look out for ourselves first.
The JW religion feeds the narcissistic part of the human personality by constantly reminding it's members that they have an inside track with God and will be the only survivors of his wrath. They convince their members that it's loving concern for others that stirs them to do good works even though it's really about saving their own skins, first and foremost.
When a JW shuns a former Witness, it's not really about loyally sticking up for Jehovah. It's mostly an opportunity for them to feel superior and extra virtuous yet humble in a Watchtower approved way.