Did he strike only 10,000?
Radak: He killed 10,000 in Gei Melach. Divrei ha'Yamim II, 25:12 explains that he captured another 10,000 and cast them from the top of the rock, and all died.
What is the significance of capturing a rock?
Radak based on Targum Yonasan: There was a fortified city there.
Why did he call it Yakse'el?
Rashi: It blunted his teeth 1 , like it says in Divrei ha'Yamim II, 25:14. After Amatzyah struck Edom, he bowed to their gods. The Navi rebuked him, and said "I know that Hashem counseled to destroy you" (15:16) - i.e. He enticed him to fight Melech Yisrael.
Radak: They had a known reason for this.
The verse implies that Amatzyah gave this name. Why would he give a name that recalls his downfall? (PF)
