Short answers, pulled from the story.
The Ustaše Croatian Revolutionary Movement was founded by Ante Pavelić and journalist Gustav Perčec in late 1929 or early 1930. This group claimed that Croats and Serbs were separated by an unbridgeable cultural gulf.
Ministers Mile Budak Mirko Puk and Milovan Žanić outlined a strategy to kill one-third of the Serbs expel one-third and forcibly convert one-third to Catholicism. Historians agree the Ustaše applied this formula systematically toward Serb communities despite never officially publishing it.
Croat demographer Vladimir Žerjavić estimated total Serb deaths in the Independent State of Croatia as 322,000 including 78,000 civilians killed in prisons pits and camps. Current estimates for the Jasenovac-Stara Gradiška complex put the number around 100,000 inmates perishing there with half being Serbs.
On the 28th of April 1941 approximately 184 to 196 Serbs from Bjelovar were summarily executed following arrest orders by Slavko Kvaternik. This event marked the first act of mass murder committed by the Ustaše upon coming to power.
The Catholic Church in Croatia headed by archbishop Aloysius Stepinac greeted the law on religious conversions and adopted it into internal church regulations. Many fanatical Catholic priests joined the Ustaše blessed their work and participated in killings and forced conversions while over 150 Serbian Orthodox priests were killed between May and December 1941 alone.