Some reasons Christian metal exists. Because some Christians like metal, but don't want to hear the lyrics put forth by secular metal artists. And the other is an opportunity to reach out through the music to the secular culture.
I've been listening to various forms of Christian metal since i started listening to Stryper at 13, before i got saved. And after i got saved my interest in metal went from a handful of hair metal bands to exploding into most areas of metal. I actually got More into metal after getting saved. I love the sound and feel metal produces, and no, i don't hear anger in all metal. True, there can be, but to me metal is not only an angry or evil thing. If anything Christian metal artists are the modern day Psalmists. Psalms is full of praise, but also crying out to God through tears of suffering. Most modern 'Christian' music is all about happy faces and feel good vibes, and less about the honesty of the human condition. The struggles, the honest emotions. Most modern Christian music to me is empty and trite and has no true expression of anything.
Christian metal can produce worship, or can hep you feel that others out there identify with your hurts and struggles. Or it can just be entertaining and fun. Its a varied and multifaceted tool God can use and, i for one, am grateful to have had it, and its influence, in my life. God has used it to help me in many ways for the past nearly 25 years. So good luck trying to take that away from me.