They come along and attach themselves to a living being and proceed to take all they want from that life while contributing nothing except disease and, potentially, death. This is what we recognize in the natural world as a parasite.
Like the parasite, the narcissist attaches itself to its host. It has identified what it sees as a source of life, a source of narcissistic supply. If their parasitism manages to suck the life out of the host they will quickly attach to the next one. There is no expression of concern or feeling for the diseased or dead host they left behind.
Narcissists...like a tapeworm or a malarial protozoan....feel no regret for this way of making a living. We should feel no regret if we choose to use pesticides to get rid of them.