if the kidnappers were paid, what is the point of killing him after that

This is very sad news but most of the time this is the end result of a kidnapping whether the ransom is paid or not, if this was a case where the kidnappers knew his victim then it is likely they never had the intention of releasing him since then they will be found by the police afterwards and since there is a lot of money involved this will surely become a high profile case where the police will do everything to find them.
Now there are have been also cases where the kidnappers did not meant to kill the victim but they did anyway or maybe they are just that cold and did it because they wanted, after all they are kidnappers and once they do that then I suppose those people do not have high morals.