Genesis 6:5

And the Lord saw that the wickedness of man was great in the earth, and all the imagery of the thought of his heart was only evil every day.

כי רבה רעת האדם, a reference to the past,

וכל יצר לב האדם, a reference to the future (when it would not improve)

'וירא ה, seeing that the time G’d had allocated to the people to improve their ways had elapsed, G’d saw that nothing had changed. רק רע כל היום, the words וכל יצר, refer to both man’s urges, the urge to do good, and the urge to do evil. The urge to do good had succumbed to the evil urge, completely. The Torah here calls the planning of wicked deeds based on impulses יצר לב, the urge of the heart. The reason is that the heart is perceived as the origin of all impulses be they good or be they evil in nature. The reason why G’d gave them 120 years extension, although He was well aware that they would not do teshuvah, was only in order for man to learn from G’d’s attributes, and to adopt this attribute for himself. Just as G’d allowed man plenty of time to improve his ways, so man, when dealing with his fellow, should also not be impatient, but allow enough time for people to rethink their evil attitudes.
When G’d created the human race He wanted it to be good completely, or at least predominantly. If mankind would turn to be completely evil it could not endure, seeing that G’d had chosen the good. When He saw that the generation preceding the deluge was thoroughly evil, especially in their inter-personal relations, their use of violence as a legitimate tool to gain their ends, their deeds threatened to undermine the foundations upon which G’d had built His universe. He therefore decided to destroy all those who were evil and to save only the few good ones, so that these survivors could form the nucleus of a better human race after the deluge. G’d had found Noach, his sons, and their wives to be good and they were chosen to provide the seed for future generations. Lemech, Noach’s father had died already 5 years prior to the deluge, and Metushelach, Noach’s grandfather, another good man, had lived his life to the full before the onset of the deluge. As a result of these two men having died, there were no righteous people left on earth other than Noach and his family.