What should be done to rapists?

Capturig a person you think guilty of a wrongdoing is not the same as them being guilty of the wrongdoing. That is why there is a system of laws and trials in most civilzed nations. Lynch law is generally frowned on.

In some cases the person is guitly of a crime, in others mentally disordered mind, in others innocent. People capturing the person have no systematic way of assessing the situation.