I know there are some heinous acts in the OT, and it's hard to get past those things for some. It's hard to understand why God would allow so much murder to occur, but you have to remember that all of the nations outside of Israel saw them as a force to be reckoned with. They were threatened by them, and knew based on how God had delivered them from slavery from the Egyptians that they were a nation in favor with God. If God didn't allow His people to attack and overtake those nations, then those nations would attack and kill His people. Then you have to think about what is the central theme of the entire Bible...Jesus. In the OT, everything is based around the theme that the Christ, the Messiah is coming. In order for that to happen, God had to preserve His people, by which Jesus would come...specifically the tribe of Judah. If God didn't protect His people, then they would have been destroyed...no lineage that was prophesied for Jesus to come. Also, the reason God allowed His people to completely destroy nations was because they were involved in pagan worship...idolatry. God didn't want His people to have any of that influence to worship false gods around them. However, we know what happens...they disobeyed Him and on multiple occasions, they did not completely destroy the nation they were overtaking. They even married women from those nations which God had given them the command to not do. Because they did, they ended up being influenced by the outsiders to worship idols, which God despised.
The central theme surrounding Jesus continues in the NT. In the 4 gospels, it's all about Jesus is here. Then Acts through Revelation is about Jesus is coming back. I hope this helps you a little. Those acts in the OT are brutal, but were necessary to preserve God's own special people, whom His beloved Son would come from.