1)
Why does it say "Gazal Ach"?
1.
Radak: All of Yisrael are brothers, e.g. "Ad Drosh Achicha" (Devarim 22:2).
2.
Radak citing Targum Yonasan: He stole from one of his brothers.
2)
What did he do that is not good?
1.
Radak: It is as if he said 'he did evil.' Verses often say "Lo Tov" in place of Ra, e.g. "Yisyatzev Ad Derech Lo Tov" (Tehilim 36:5), and similarly "Asher Einenah Tehorah" (Bereishis 7:8).
3)
What is the Chidush of b'Soch Amav?
1.
Radak: He is not ashamed to do evil amidst congregations of people.
4)
Why does it say that he died for his sin?