Quoting from the official Ishbitza-Radzyn website:
Rabbi Shmuel Shlomo Leiner eventually took over for his father, the Tiferes Yosef, but it took time and a lot of people to convince him. He was very zealous. He held high standards for Radzyner shuls and would close down any that didn’t meet these standards, e.g. one in which the Gabbai didn’t send his daughters to the local Beis Yaakov school but sent to the public school instead. In spite of efforts to bring him to the USA, he elected to stay with his chassidim. He encouraged his chassidim to fight the Nazis, allowing Jews to escape to the woods. Eventually the Nazis found out who had been causing so much trouble for them and hunted him down. He was executed on the 29th if Iyar in 1942. He went down with a fight, spitting at his Nazi captors.
Or on Wikipedia:
The Rebbe was known for encouraging resistance to the orders of the Nazis and the Judenrat and for urging people to break out of the ghettos, flee to the forests and take up arms. (Although he never made it to the woods himself, his brother-in-law, Rabbi Avraham Yissachor Englard, fled to the forest and joined the partisans with a group of Radziner Chasidim.) The story goes that in his last moments, the Rebbe was wrapped in his father’s tallis as a German soldier took him into the Włodawa cemetery at gunpoint. At one point, the Nazi pushed him. The Rebbe turned around, slapped him across the face, and kicked him. This obviously degraded the Germans greatly, thus stunning him by showing the inner power of the Jewish people. The Rebbe was immediately shot to death on that day, 29 Iyar 5702 (1942). Buried in Włodawa.
And see this.