Short answers, pulled from the story.
Ante Pavelić led the Ustaše as Poglavnik from the organization's formation through the collapse of the Independent State of Croatia in 1945. He fled to Argentina via Austria and Rome with the help of Catholic clergy. On the 9th of April 1957, a Montenegrin named Blagoje Jovović shot him near Buenos Aires; Pavelić later died of his injuries.
The Ustaše killed an estimated 30,000 Jews and between 26,000 and 29,000 Roma. Estimates of Serb victims vary widely; the United States Holocaust Memorial Museum places the most reliable figures between 330,000 and 390,000, with 45,000 to 52,000 Serbs murdered at Jasenovac alone. The Jasenovac Memorial Area estimates roughly 80,000 to 100,000 people were killed in that camp complex.
The Independent State of Croatia (Nezavisna Država Hrvatska, NDH) was proclaimed on the 10th of April 1941 after the Axis invasion of Yugoslavia. It was an Italo-German quasi-protectorate divided into German and Italian zones of influence. The state collapsed in 1945 with the German surrender and the establishment of socialist Yugoslavia.
On the 9th of October 1934, Ustaše operatives working with the Macedonian revolutionary organization IMRO assassinated King Alexander I in Marseille, France. The gunman, Bulgarian revolutionary Vlado Chernozemski, was killed by French police at the scene. Three Ustaše members who had been waiting elsewhere in the city were captured and sentenced to life imprisonment by a French court. Pavelić was sentenced to death in absentia as an organizer.
Archbishop Alojzije Stepinac issued a public letter supporting the Ustaše state on the 28th of April 1941 and served as the state's War Vicar, receiving a medal from Pavelić in 1944. Some Franciscan clergy participated directly in atrocities, including friar Miroslav Filipović, who joined in the massacre of roughly 2,730 Serbs on the 7th of February 1942 and later became Chief Guard of Jasenovac concentration camp. After the war, Catholic priests ran ratlines that helped smuggle Ustaše leaders to South America.
Jasenovac was the largest concentration camp operated by the Ustaše, built between August 1941 and February 1942 in the Independent State of Croatia. Its sub-camps included Stara Gradiška, designated for women and children. The Jasenovac Memorial Area maintains a verified list of 83,145 named victims and estimates the list represents 60-75% of total victims, placing the number killed in the complex at roughly 80,000 to 100,000.