Anyone who is a serious follower of Jesus and who stakes his or her life on the God revealed in Scripture has wondered, "What's up with all the 'anger of God' stuff in the Old Testament?"
Here's my finite, fallible take on the topic: When Holy God reveals himself to a fallen people in a fallen creation in order to establish his presence among them for all to see and know, the clash of God's holiness with the fallen will cause conflict and anger on both sides of the relationship.
The world was and is a fallen mess. Insert Holy God who wants to love and guide a fallen people in a fallen world, and sparks will fly. Here's an analogy:
Try choosing to love a 16-yr-old, rebellious street kid who had scratched and clawed to survive her whole life and of whom many people had taken advantage. She is "owned" by a man who pimps her for his profit. Upon her rescue, you would have to fight those who thought they still owned her--taking some very, un-Christian actions against them. After she is in your care, you would have bouts of anger with her as she tried to survive the ways she learned on the streets while learning to trust you--who she had only known in name until you showed up and bought her out of the brothel where she lived/worked. If you have ever tried to rescue a child and to protect him or her from those who had enslaved them and to regain his or her trust, you may a glimpse of God's anger motivated by love toward those who refused to worship the God who had rescued them from Egypt.
Another take that sheds some light on God's "anger" for me is the fact that when the holy, eternal God seeks to enter His creation in order to rescue it, one ignites the other. Holy God does not thrive in fallen creation. It's against his nature and being. However, in an effort to enter Israel's world so He could lead them, God gave Israel the tabernacle, for example, with all its ornaments and altars along with priests set apart to serve Him so people would see both His separateness and approachability, but that is like introducing electricity to a swimming pool. (Thus, the rules about not touching stuff and standing at a distance and killing an animal as a sacrifice.) What was once a safe place to lounge for vacationers, (or, for people to live and work ) is now a danger zone in which those who formerly swam there would die if they dove in unaware. God's literal presence electrified creation, and those who did not respect what He had ignited paid with their lives.
There is much more to be said about this--especially by those more astute in theology than me, but these two concepts help me at least understand a bit about how a loving God could appear so mean to the street kids He has come to rescue.