First, I would thank your wife for the courage to admit that she has problems with portions of the Bible, in particular the Old Testament. She is not alone. Second, I would suggest that she needs to fill in big gaps in her knowledge of the Bible's contents. The harsh features she cites are minimal compared to the Bible's revelation of God as loving, just, righteous, compassionate, and kind.
Those descriptive terms are consistently true of God's actions, notwithstanding the fact that He could, and, when necessary, would wipe out whole groups of people.
His seemingly harsh justice in ancient times must be understood within the framework of a fallen (that is, universally corrupt) humanity and fallen angels (that is, demons). The groups of people God wiped out were not bucolic rustics, such as those who lived only in Rousseau's imagination. God could not tolerate their hopelessly depraved and corrupting existence indefinitely. His holiness demanded justice, like the Flood.
The apostle Peter describes (a) the sending of rebellious angels to pits of darkness, "reserved for judgment"; (b) the sending of the Flood "upon the world of the ungodly"; and (c) God's condemnation of the cities of Sodom and Gomorrah to be "an example to those who would live ungodly lives thereafter." Our generation needs to read 2 Peter 2 as a corrective to the notion that God is a cuddly little God who is only mildly upset by moral corruption. Peter says striking things about divine judgment, as do the other writers of the New Testament. They saw judgment as inherent and inevitable in a world that opposed God.
The warnings given by Peter are expressed in terms that are consistently true of God. He is holy and cannot stand by and watch His creatures become depraved enemies of all that is good. In the past, His holiness demanded the seemingly harsh justice of Old Testament times. On the other hand, His love demanded provision of a Redeemer and Savior of all who come to Him in repentance (see Micah 7:18).
Finally, it is not our prerogative to define divine features, such as kindness, or to determine what a kind God would or would not do. Questions asking whether God is kind or not kind are answered by Him in the Bible, not by us. So we return to the second suggestion in this attempt to respond to your wife's observations about kindness and the God of the Old Testament: we need to read the Bible more consistently and deeply.