Book of Joshua
It describes the Israelite conquest of Canaan by its eponymous hero and takes place after Moses freed the Hebrews from Egyptian bondage and led them through the wilderness for 40 years. The book ends with Joshua's dividing up the territories between the 12 tribes and his subsequent death.
Some have considered it one the more troubling books of the Bible due to its apparently unambiguous exhortations to the murder of civilians - for example, after Joshua has put the entire population of Jericho to the sword (Joshua 6:20), God instructs him to 'Do to Ai what you did to Jericho' וְעָשִׂיתָ לָעַי וּלְמַלְכָּהּ כַּאֲשֶׁר עָשִׂיתָ לִֽירִיחֹו(Joshua 8:1).
However, one should be careful to distinguish between murder for murder's sake, and being an instrument of God's wrath - the Canaanites at the time were practitioners of child sacrifice and other abominations and God told the children of Israel to wipe them out lest they be corrupted. They failed to carry out his instructions, and corruption away from God was a constant problem in their history.