Unfortunately,being in a church doesn't immune anyone who has free will and chooses to do evil acts instead of good acts. Its no different than any other group or other professions who also have had some who choose to do evil deeds when most in those groups or professions, do not.
No, this man was not a true Christian for if he truly was then he would know killing anyone is a grave sin, and would not have done so.
It sounds like he pushed God away, and decided to do the devil's work instead.
My prayers are with the true Christians in his church would must be reeling from what he has so wickedly done to his innocent family. My prayers are also with his victims' families as they go through their tremendous loss.
I was just reading about a Pastor who impregnated a 14 year old child. He is no Christian either and only used it as a ruse to do his evil. Evil is everywhere in all professions. He is no different than the predatory teachers, coaches or others who purposefully choose a profession so they can be around, and prey on children. Here is the article on the Pastor if anyone is interested.
http://www.foxnews.com/us/2017/11/2...-allegedly-impregnating-14-year-old-girl.html