Malignant narcissists operate very much like slithery snakes. They pretend to be your friend and they like to elicit your help. I recently saw an illustration on a social media site that reminded me of how they work. It showed one person helping another, trying to untangle the noose around his neck. Both men were resting on stools. As the helper was freeing his "friend," unbeknownst to him, the one entangled by the rope also had his foot on the other person's stool, ready to kick it out from under him.
A morally disordered person will still try hurt you, even though you've just done something nice for them.
This reminded me so much of my encounter with a malignant narcissist. I was always ready to help her, because that's what friends do. Because I felt sorry for her (morally disordered people like to play upon your sympathies) I was empathetic to the fact she complained about having little money and being deeply in debt. So, on a couple of occasions, I gave her $20 to buy some groceries. I'd also give her children clothes that my children had outgrown.
Narcissists love it when people help them, and they obtain this help by throwing out little hints that make you feel obligated to respond to their needs. They fully expect that you will behave the way they want you to. They expect all kinds of favors because they feel entitled to them. (This is one of the characteristics of their disorder.) However, at the same time you are performing an act of kindness, they are busy plotting your destruction. You can never trust a narcissist.
They take whatever you will give them, even as they are planning evil deeds against you, or getting ready to discard you. Don't feel too badly if you have ever been taken in by a malignant narcissist. Until your eyes are opened to this insidious condition, typically from first-hand experience, you could never even begin to fathom that someone who seems so righteous could be so ruthless.