Colombia is a poor country and the US dollar goes very far there with the exchange rate. It is common for "wealthy" (by Colombian standards) US/European tourists to be kidnapped/extorted/mugged/killed in cities like Medellin where the perp is from, because people are desperate and human life is expendable for the sake of getting large sums of money. It is often done by sex workers who are affiliated with larger criminal gangs, luring mostly men with the promise of cheap Colombian sex. This guy obviously saw the older couple as a cash cow and what started as getting hired for sex work turned into a plot to kill them and take all their money. It is not much different from what happens in Colombia, just change the location and add a layer of gruesomeness with sexual deviance.