Take a look again at God's mercy

God is merciful. Even His chastisement is merciful.
In the Old Testament, God followed a certain modus operandi in dealing with the Israelites.
After crossing the Red Sea, God brought the Israelites to Mount Sinai to establish a covenant with them. God wanted to make the Israelites a kingom of priests, a holy nation (Exodus 19:6a). God called Moses up Mount Sinai to converse with him for 40 days to give him the Ten Commandments. However, the Israelites below grew impatient and insecure. They forced Aaron to make them a golden calf for worship and led them back to Egypt! Naturally, God was mad with them. He punished the Israelites by sending them into the wilderness to wander for 40 years before entering Canaan, an Exodus which would have lasted no more than a few months. The objective? I guess God wanted to cleanse the Israelites of the polluted elements which they carried with them out of Egypt. Thus, only a new generation of Israelites, uncontaminated by the idolatry in Egypt were admitted into Canaan

Again, the southern kingdom of Judah was conquered twice by Babylon because all the people ignored the warnings from the prophets and fell for idolatry. They were twice exiled to Babylon. When Cyrus, the Persian king, in turn conquered Babylon and decreed that the Jews should return to Jerusalem to rebuild the Temple of God, it was 70 years (50 years) later (2 Chronicles 36:21). Those Jews that returned must be a new generation of Jews. The objective? Again, I guess God wanted a new creation.

When a country is conquered by another, there must have been a lot of bloodshed and atrocities. Many people, including innocent civilians suffered terribly. So, how does such cruelity square with the mercy of God? It is always difficult to defend God's actions. However, we should appreciate the fact that God did not punish to the fullest extent. Consider the following verses against idolatry.
you shall not bow down to them (man-made idols) or serve them; for I the LORD your God am a jealous God, visiting the iniquity of the fathers upon the children to the third and the fourth generation of those who hate me, 
but showing steadfast love to thousands of those who love me and keep my commandments. (Exodus 20:5-6)
Nowadays, we take 30 years as one generation. Remember this is a modern number. In ancient times, perhaps a girl would become a mother at 13. So, let's take 15 years as one generation. Therefore, 3 generations would be 45 years and 4 generations 60. Therefore, God is faithful and keeps His promises, even in His punishment! The Israelites wandered for less than 3 generations before entering Canaan and the Jews were exiled for less than 4 generations before returning to Jerusalem to rebuild the Temple.
If you are still not satisfied with God's leniency, let me give you one more example from Genesis.
God warned Adam that he would die the day he ate the fruit from the tree of knowledge of good and evil (Genesis 2:17). Yet Adam died at the age of 930 (Genesis 5:5). Did God tell lie? Of course not. It is because God lived in a different time frame. For Him, a thousand years are no more than a day (Psalm 90:4). So, God still keeps His words and He has been very lenient to man.

God would have been indifferent. He would have created the universe, left us there and let the cosmo run according to some natural laws. He would have been a distant and reticent Creator. Yet, He continually intervened in human history. It shows that He cares. More than that, He loves by sending His only Son, Jesus, to die for us to redeem us.
For God so loved the world that he gave his only Son, that whoever believes in him should not perish but have eternal life.
For God sent the Son into the world, not to condemn the world, but that the world might be saved through him (John 3:16-17).
God loves. God does not condemn. We do not love ourselves. We eat the cake we bake.
Even though God is merciful, we should be prepared to suffer the consequences of our sins. Don't blame God for our sufferings.

Dear Lord, in Your mercy we trust. Motivate me to repent, to confess and to seek reconciliation. Amen.