Mr. de Kuijer, a former European Commission diplomat, was also en route to the AIDS conference. A couple of years ago, he famously came out as being gay during a stand-up comedy routine in a well-known cafe in Amsterdam. After that experience, he committed himself to activism of all sorts. His passion for equal rights and democracy even brought him to Ukraine as a foreign election observer during the May 25 presidential election, after the Ukrainian revolution that brought down pro-Russian leaders. Mr. de Kuijer watched the chocolate producer Petro O. Poroshenko win the vote and was pleased, a close friend and former employer said. "Pim was so proud to contribute to democracy in the Ukraine,” said Lousewies van der Laan, a Dutch politician living in Slovenia. Rafal Farbisz, who worked with Mr. Kuijer in Sierra Leone, described him as a "friendly, passionate traveler and backpacker." Hundreds of people were expressing disbelief and sorrow on a Facebook post that Mr. Kuijer put up from Amsterdam's Schiphol Airport before boarding the flight.