1)
Why did Hashem say this to Yechezkel?
1.
Malbim: Initially Hashem told him "ha'Nakel l'Veis Yehudah? Ka Mal'ah ha'Aretz Chamas", and they send the Zemorah to cut off the nation (8:17). Now he explained that the officers are the ones who think and counsel evil.
2)
Why does it say "veha'Yo'atzim Atzas Ra"?
1.
Radak: They were officers and leaders of the city. The counsel of the city depends on them ? evil counsel.
2.
Malbim: They counseled not to make Shalom with Nebuchadnetzar; the city wall will protect them ? the enemy cannot conquer it.