If the Muslim changes to Christianity or ethiesm or what ever and start attacking Islam and spoiling Muslims then the first thing is to sit with him/her and if he/she still not changed and keep attacking Islam and spoiling Muslims then I would go with opinion which say that punishment is death.
Death is more merciful then sending him/her for life in prison, imagine he/she is 25 years old and the time he/she spend in prison is between 40 and 50 years (not a short period).
Also if this person who is sent to life in prison might be freed after 20 years and there might be a possibility that he/she will attack Islam again and spoiling people, so why to keep a probability for spoiling people?
Please note that Islam can not be compared to great Treason ( conspiracy)