IMO, religion, has no relavence in this case. The church just happend to be a space that MB either rented out or was allowed to use, most likely in exchange for some sort of donation to the church. May just because the church is a charritable organization that supports their community, member or not. Regardless, MB had other venues to work out that were not churches. A church, letting a member, or member of the community use its facilities, is not at all unusual. As MB was not a member ot this church, I doubt that many, if any, COC members knew of her marital indiscretions, nor do I think, regardless of what their beliefs are that a fanatic may have killed her because he/she disliked that she was using the facility for a work out class.
I believe it was the most convient location for the crime to be commited. A little remote, alone, wee hours of the morning. I am not of the opinion that this person had to be someone in MB's inside circle (family or close friends) to know that she would be there that morning, as one of the first facts we learned was that she announced it to the world on facebook the night before. I have no doubt that most likley this suspect knew MB in some regard. What that was, I have not idea, but I do have my suspicions. Im not in the Mulitple perp, family did it camp.
As for the question of what kind of person would do this in a church? Well, the unfortunate facts, are that population in general, have become less and less religious, and the majority of our young people are now either agnostic, or out and out athiest. Many of our adults are agnostic, and religion really just has no relavence in their lives, so Church, burger joint.....nickles worth of differance, and particularly to the type of personality that would kill someone.