“[…] Do not let your eye feel sorry, and do not feel any compassion.”
~Ezekiel 9:5

Is it cruel on God’s part to destroy the wicked?
Jesus said God executes justice “speedily.” (Luke 18:7,8)
But when Ezekiel prophesied about Jerusalem’s destruction, five years had yet to pass before its fulfillment.
Since the days of Moses, Jehovah had sent one prophet after another to warn his people of what would happen should they stray from true worship. (Jer. 7:25)
Still, the people as a whole were not destroyed.
Jehovah examined them individually and figuratively marked those who would survive Babylon’s invasion.
Furthermore, the religious leaders most responsible for corrupting Jehovah’s worship would be the first to be executed. (Eze. 9:6)
This teaches us that being a nominal Christian or getting baptized is not enough to receive the “mark” of salvation.
Jehovah and Jesus will closely examine who among us truly has faith by our acts of worship. (Jas. 2:24)