1)
Why did he say that Hashem did this to you (tore his kingship away) today?
1.
Radak (15:28) #1: Hashem said so after the sin of Amalek, even though Shmuel already told Sha'ul so at Gilgal, when he did not wait for him (13:14)! The first sin could have been pardoned through Teshuvah and good deeds. "Today" (after the sin of Amalek) it was [irrevocably] torn away.
2.
Radak (15:28) #2: Before, Hashem decreed that the kingship will not pass to Sha'ul's children, but he himself could reign a long time. "Today" (after the sin of Amalek), Sha'ul will not reign a long time.
