The flood of Noah:

Genesis 6:5-9:

"And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And it repented the LORD that he had made man on the earth, and it grieved him at his heart. And the LORD said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them. But Noah found grace in the eyes of the LORD. These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God."

God is described as having created the earth, mankind, other living things, and the rest of the universe in the early chapters of Genesis. But he apparently was unable to foresee the future behavior of his creations. In particular, He did not predict the degree of wickedness that mankind would exhibit. He regretted his decision to create mankind. So he decided to commit the ultimate act of genocide, by murdering the entire human race: men, women, children, infants and newborns. God decided to exterminate people by drowning - a slow and painful way to die. He allowed Noah to survive, along with Noah's wife, his three sons and their wives. But the Bible states that the rest of the human race were wiped out, including young children and infants who had not reached the age of accountability.

In the 20^th century, the most serious acts of genocide involved less than 1% of the human population. Examples are: the extermination of the Armenian minority in Turkey, the extermination of Jews, Roma (Gypsies) and others by the Nazis, the extermination of the ethnic Albanians by the Serbs in Kosovo in the former Yugoslavia. The perpetrators have become the most hated of people. But the genocide resulting from the great flood is far more serious. It is recorded as having destroyed over 99% of the human race, leaving only eight humans alive. 

Genocide of the residents of Canaan:

The Israelites invaded Canaan and, under God's instructions, exterminated seven nations in widespread acts of genocide: the Girgashites, Amorites, Canaanites, Perizzites,  Hivites, and Jebusites. They continued to commit genocide against other groups.

Deuteronomy 7:1-2:

"... the seven nations greater and mightier than thou; And when the LORD thy God shall deliver them before thee; thou shalt smite them, and utterly destroy them; thou shalt make no covenant with them, nor shew mercy unto them."

Joshua 6:21:

"And they utterly destroyed all that was in the city, both man and woman, young and old, and ox, and sheep, and ass, with the edge of the sword." 

This latter passage describes one event in the invasion of Canaan by the ancient Israelites. After the walls of the city of Jericho fell, the soldiers ran into the city, and murdered all its inhabitants: elderly men and women, mature men and women, pregnant women, youths, boys, girls, infants and newborns. Their goal was to entirely wipe out the Canaanite culture by destroying its people; this is one definition of genocide. Incidentally, the people were butchered by the edge of the sword, because the weapons did not have pointed ends.

Joshua 10:40-41:

"So Joshua smote all the country of the hills, and of the south, and of the vale, and of the springs, and all their kings: he left none remaining, but utterly destroyed all that breathed, as the LORD God of Israel commanded. And Joshua smote them from Kadesh-barnea even unto Gaza, and all the country of Goshen, even unto Gibeon." 

As recorded in Joshua 11:19-23, God had "hardened the hearts" of the Canaanites, so that all but one city attempted to fight the Hebrews in battle. The sole exception were the Hivites, the inhabitants of Gibeon. "As the LORD commanded Moses", all of the rest were defeated in battle; their cities and populations were destroyed. This included people of all ages: men, women children, infants and newborns. 

Genocides and other extreme atrocities are recorded in: