Why do people have a problem with God killing people?

But not have a problem with humans killing one another senselessly in wars. Unlike humans God can read hearts. He knows when a person is just wicked and not going to change. But humans can't read hearts. They kill to support ones country or for stupid things like oil. But you will cheer for humans but blaspheme God. God is the source of life. He created us. So therefore he has the right to take a life if he chooses. What gives humans the right to take a life?