When we have a goal to reach, we often do things differently during different steps we must take to get the job done . . . Consider the gardener . . . First he must remove all of the weeds, then he adds appropriate amendments where needed, and only then does he begin to plant & water & landscape to create a beautiful garden! Well, think of Jehovah as the gardener, the earth as the garden, and pagan cultures as the weeds. God removed them so that his chosen fruit trees could eventually take root & thrive, without their deathly competition!
The OT shares the history of people's relationship/s with --including an extreme rebelliousness toward-- Almighty Jehovah God. They often became so violent & depraved that God destroyed entire towns, tribe, and even everyone except one family of 8 in the Flood of Noah's day!
For example, the Canaanites were *exceedingly* savage with their *own* babies, among other horrible practices:
"...Immorality, pagan worship, and child sacrifice were widespread in Canaan.
Bible historian Henry H. Halley notes that archaeologists excavating the area
“found great numbers of jars containing the remains of children
who had been *sacrificed* to Baal [a prominent god of the Canaanites].
“The whole area proved to be a cemetery for new-born babes. . . . Canaanites worshiped, by immoral indulgence, as a religious rite, in the presence of their gods; and then, by *murdering their first-born children, as a sacrifice* to these same gods. It seems that, in large measure, the land of Canaan had become a sort of Sodom and Gomorrah on a national scale. . . . *Archaeologists who dig in the ruins of Canaanite cities wonder that God did not destroy them sooner than he did!”...*
"Why Did God Wage War Against the Canaanites?"
http://wol.jw.org/en/wol/d/r1/lp-e/2010006
"Other practices of the Canaanites were:
spiritism with its seances, casting of spells, fortune-telling (De 18:9-12),
offering children --as LIVE BURNT OFFERINGS-- to their gods (De 12:31; Jer 32:35; 2Ki 16:3),
incest, sodomy, and bestiality. (Le 18:6, 22-30; 20:13) . . .
"The Canaanites also practiced “sacred” prostitution by male and female temple prostitutes . . .
De 23:17, 18; 1Ki 14:24.
"On the basis of these and other “abominable” or “detestable” practices, Jehovah God ordered Israel to devote the Canaanites to destruction so that no contamination by false religion would result. (De 20:17, 18)
"Any Israelite practicing the same things or advocating such apostasy was to receive *precisely the same* penalty.—De 13:12-15; 17:2-7; Ezr 9:1, 11-14.
.
"Detestable Thing"
http://wol.jw.org/en/wol/d/r1/lp-e/1200001161?q=canaanites&p=par
Not only were the pagan cultures steeped in such practices, they were living on land that God had earlier promised to the extended family of his friend, Abraham . . . and, they were NOT inclined to give it up to them!
God was preparing a place & a people among which his son --Jesus Christ-- would be born & raised. but --eve though he took the precautions of removing pagan religion & its followers from the land-- the Israelites still succumbed to it, and after warning them & them ignoring him further, he allowed them to be removed (many died in the process) for a set amount of time, after which he saw to it that a remnant of their offspring were allowed to return. Thus, Jesus Christ was eventually born in Israel. . .
Jesus began teaching several new concepts to God's people, but the vast majority rejected him, having him killed! This, even though the hundreds of prophesies about him only his life fulfilled, proving him to be their Messiah! Yet, during his short ministry, Jesus taught that a few of his faithful followers would actually be reassigned to rule alongside him in heaven over humankind & the earth, after Armageddon; that God would henceforth not require them to kill anyone under any circumstance. Jesus had come & would soon leave the earth, and from then on the pagan nations would be allowed to do as they please . . . up until Armageddon, when they will be annihilated for all time.