If she took the lives of 6 innocent people, then she deserves to have her life taken away from her. 19 is old enough to be completely responsible for your actions.
However, I am not for the death penalty. I say life in prison with no chance of peroll.
I think a life sentence is 25 years? Well 25 x 6 = 150 years she should be behind bars. If she reaches that age (169), by all means, free her.