Why do merfolk fall in love and then kill people?

Answer 1

Scientifically merfolk aren't real.

This is not a trait common to all merfolk. The incidences of merfolk luring sailors to their death is equivalent to those humans who host cock-fights and dog-fights. It doesn't represent the attitudes of the entire race but because it is of the few incidences in which merfolk and humans interact it has become their most well-known trait.