I suppose you don't understand the context for all this happening. It's not obvious because we've lost the context for all these stories as written, as the people for who this is written for knew the context without being told because it was part of the culture. 

For example, even if someone isn't super familiar with all the rules of American football, everyone knows enough about it through cultural exposure to have a context for teams and leagues and whatnot. 

The context for these stories is that there are actual humans (descended from Noah and then you have the Isrealites that came from Abraham) vs. the corrupted contingent of human-spiritual being hybrids that were compromised in their worship of false gods via their practices which were all canabalistic and perverted. 

So, all the tribes that God calls the Isrealites to eliminate are all cannibalistic, child-sacrificing groups. And I would agree with God that they should be eradicated.