When did they conspire against him?
Rashi: It was from when they heard the Navi say "Ya'atz Elokim Lehashchisecha" (Divrei ha'Yamim II, 25:16) 1 . It was 15 years before he died 2 .
Radak: It was just before he died. Divrei ha'Yamim (II, 25:27) says "from the time that Amatzyah veered from Hashem" the evil came upon him. He was stricken in war, and his situation worsened every day, until "they conspired against him", and he fled and they killed him. The verse mentioned only the final evil that befell him.
Malbim: [Perhaps] it was three years after his war with Yo'ash, and 12 years before he died. Initially the conspiracy was covert. It started after his war with Edom. Perhaps the conspirators gave him the bad counsel to fight Yo'ash, in order that he fall to him 3 ! After he ruled another three years in Yerushalayim, the conspiracy spread, and he fled to Lachish.
This was after he served Edom's idolatry, and [it seems] before he fought Melech Yisrael. (PF) 'Rashi' (Divrei ha'Yamim II, 25:27) - it was after he fought Melech Yisrael.
Radak: The verses imply unlike this. It says that after they buried Amatzyah, they made his son Uziyah king; he was 16 at the time. How did he rule 15 years in his father's lifetime?! The verses in Melachim and in Divrei ha'Yamim do not imply that this was when Amatzyah fled. Also, how does it say that Amatzyah reigned 29 years? It was only 14! If he was hiding in Lachish, this is not ruling! (Also, verse 2 said that he ruled 29 years in Yerushalayim! - PF) Also, the verse says that his son ruled only 52 years, and they were after his father's death. (Perhaps the verse (15:1) includes the years he ruled in his father's lifetime! - PF) If you will count the 15 years for his son, he ruled 67 years! Also, "va'Yanas Lachishah va'Yishlechu Acharav" implies immediately after he fled!
Surely they realized that if he falls, many Jews from Yehudah will fall with him! Perhaps they reasoned that this is better than allowing him to lead Yehudah to abandon Hashem and serve idolatry again! (PF)
Why did he flee to Lachish?
Malbim: In Yerushalayim were Kohanim and Ovdei Hashem, Talmidim of Yehoyada. In Lachish he found supporters all these years.
'Rashi' (Divrei ha'Yamim II, 25:27): It was a fortified city. Sancheriv fought against it.
'Rashi' writes that they killed him 15 years after he fled. Who ran the Medinah during this time?!
Rashi: His son Uziyah reigned for those 15 years.
'Rashi' (Divrei ha'Yamim II, 25:27): Yechalyah, the mother of Uziyah, reigned then.
