1)
Why did he kill all of Beis Yaravam?
1.
Malbim: This was to strengthen his kingship, lest Beis Yaravam rebel against him [and try to take back the kingship].
2)
Why does it say "Kol Neshamah"?
1.
Malbim: He killed even the women and children.
3)
He killed them "ki'Dvar Hashem." Why was he punished for this (16:7)?
1.
Refer to 16:7:5:1-2 and the notes there.