Paul admonishes us to consider both God's goodness and severity. Both of these characteristics are part of what He is, and therefore we must respect both.
We can make the contrast between them very sharp: Until Adam and Eve sinned, they received all goodness from God. After their sin, it appears that His severity hit them immediately; they were ushered out of the Garden.
In Genesis 4, Cain sinned by killing Abel. At first, Cain received goodness from God in that he was permitted to live, but he also received severity in that he had to live as a marked vagabond.
In these two early examples, both sides of God's judgment appear, and a pattern has become especially clear: Severity results because of sin.
Whether we are immediately aware of it is not the issue. For instance, when Adam and Eve sinned, their decline into death began immediately, regardless of whether they were physically aware of any deterioration of their health. Cain immediately became a vagabond separated from his family roots. The only real difference between the carrying out of the death penalty between Adam and Eve and Nadab and Abihu is the effect God desired to create by His immediate, shocking display of severity.
We who draw near to God must know that the God we serve is to be served as a holy God—especially because He is a holy God. He permitted no opportunity for them to repent because they should have known better. The wages of sin is always death.
In the days of Noah, the goodness of God spared only eight souls, but His severity terrifyingly destroyed the remainder of humanity. In a matter of a few days, perhaps billions of people, even innocent unborn and newborn babies, died because of sin.
Following King Saul's paranoia-driven reign, God showed His goodness to Israel by raising up David, a man after His own heart, to rule over Israel. Yet, then David committed a disastrous double sin by entering into adultery with Bathsheba and deliberately bringing about the death of the loyal Uriah. God mercifully forgave the sins, but that does not mean there was no painful punishment laid on David and his family. God's severity against David and his family was hard and long. First, the son born of that illicit union died shortly after his birth. Later, David's firstborn son, Amnon, raped his own half-sister, Tamar. Tamar's brother, Absalom, seeking revenge, killed Amnon and fled from David, not seeing him for two years.
Then Absalom revolted against David in his desire to take over the throne. Absalom arrogantly proceeded to defile David's concubines in the sight of all Israel. Several thousand were killed in this revolt, and eventually, Absalom himself was killed by Joab. The family's pain did not stop even then, washing over into Solomon's reign when he put Adonijah to death because he sought political power by asking for Abishag's hand in marriage.
David was a man of blood, as God Himself mentions, but his children carried a moral cancer until death stopped them. Have we ever noticed how much deceit and illicit sex was involved in David's family's sins? God was not present in that family's life as He had once been. He could have stepped in at any time and stopped the holocaust ripping through David's family, but He did not. Living as they did, despite being the progeny of a man God greatly loved, they felt the severity of His judgments.
Deeply consider how long God's severity lasted! He did not simply let David off the hook. He had been chosen to draw near to God. Much was required of him.