Answer by taking a quote from the following article:

Sir Joseph Rotblat  (4 November 1908 - 31 August 2005) was a Polish physicist, a self-described "Pole with a British passport". Rotblat worked on Tube Alloys and the Manhattan Project during World War II, but left the Los Alamos Laboratory after the war with Germany ended. His work on nuclear fallout was a major contribution toward the ratification of the 1963 Partial Nuclear Test Ban Treaty. A signatory of the 1955 Russell-Einstein Manifesto, he was secretary-general of the Pugwash Conferences on Science and World Affairs from their founding until 1973, and shared, with the Pugwash Conferences, the 1995 Nobel Peace Prize "for efforts to diminish the part played by nuclear arms in international affairs and, in the longer run, to eliminate such arms."

Rotblat returned to Britain to become senior lecturer and acting director of research in nuclear physics at the University of Liverpool. Most of his family had survived the war. With the help of a Polish man, his brother-in-law Mieczyslaw (Mietek) Pokorny had created false Polish Catholic identities for Rotblat's sister Ewa and niece Halina. Ewa, taking advantage of the fact that she was an ash blonde who, like Rotblat, spoke fluent Polish as well as Yiddish, smuggled the rest of the family out of the Warsaw Ghetto. Mietek, Rotblat's brother Mordecai (Michael) and Michael's wife Manya, Rotblat's mother Scheindel, and two Russian soldiers lived in a concealed bunker underneath a house near Otwock, in which Ewa and Halina lived with a Polish family. Displays of Polish anti-Semitism that she witnessed during the Warsaw Ghetto uprising embittered Ewa towards Poland, and she petitioned Rotblat to help the family emigrate to England. He therefore now accepted Chadwick's offer of British citizenship so he could help them escape from Poland. They lived with him in London for some time before becoming established. Halina would go on to graduate from Somerville College, Oxford and University College London, and become an editor of the Dictionary of National Biography.  Rotblat felt betrayed by the use of atomic weapons against Japan, and gave a series of public lectures in which he called for a three-year moratorium on all atomic research. Rotblat was determined that his research should have only peaceful ends, and so became interested in the medical and biological uses of radiation. In 1949, he became Professor of Physics at St Bartholomew's Hospital ("Barts"), London, a teaching hospital associated with the University of London. He remained there for the rest of his career, becoming a professor emeritus in 1976. He received his PhD from Liverpool in 1950, having written his thesis on the "Determination of a number of neutrons emitted from a source". He also worked on several official bodies connected with nuclear physics, and arranged the Atom Train, a major travelling exhibition for schools on civil nuclear energy.  At St Bartholomew's, Rotblat worked on the effects of radiation on living organisms, especially on ageing and fertility. This led him to an interest in nuclear fallout, especially strontium-90 and the safe limits of ionising radiation. In 1955, he demonstrated that the contamination caused by the fallout after the Castle Bravo test at Bikini Atoll nuclear test by the United States would have been far greater than that stated officially. Until then the official line had been that the growth in the strength of atomic bombs was not accompanied by an equivalent growth in radioactivity released. Japanese scientists who had collected data from a fishing vessel, the Lucky Dragon, which had inadvertently been exposed to fallout, disagreed with this. Rotblat was able to deduce that the bomb had three stages and showed that the fission phase at the end of the explosion increased the amount of radioactivity by forty times. His paper was taken up by the media and contributed to the public debate that resulted in the ending of atmospheric tests by the Partial Nuclear Test Ban Treaty.

Why did he feel that way?
Rotblat was determined that his research should have only peaceful ends, and so became interested in the medical and biological uses of radiation.