Who is Hadoram?
'Rashi', Rashi (Melachim I, 12:18): It is Adoniram 1 [whom Shlomo appointed over the tax (4:6). It is the same Adoram that David appointed over the tax (Shmuel II, 20:24) (Radak 4:6).
'Rashi', Mahari Kara (Melachim I, 12:18): At the time of greatness, in the days of Shlomo, he was called Adoniram. Now he was lowered, so he was called Hadoram (in Melachim I, 12:18 - Adoram).
Why did he send Hadoram?
'Rashi', Malbim (Melachim I, 12:18): It was to collect the tax, to show that he wants to collect taxes with a strong hand, like before. 1
Radak (Melachim I, 12:18): It was to get Yisrael to accept Rechav'am as king.
Malbim: This was foolish. The people already hated him, for he was Shlomo's tax collector. Until now, there was still hope that Yisrael would consent to Rechav'am; now, there was no hope.
Why did they stone Hadoram?
'Rashi': Rechav'am did not know that they are rebelling against him. Hadoram tried to collect the tax that Shlomo used to take from them.
