How can you say Christianity forbids kill when there are expectations to this rule?

Thats Old Testement. I believe as a Christian that the Old Testement sets the ground rules for the old testement. I follow the New Testement. I myself dont completly understand why God did the things he did in the old testement but I will ask him that someday. I see the Old testement as History 1. And I see the New Testement as History 2 and an application testement. Hope I could help.