Based on comments BB and his family members have said, it was clear that MB's lifestyle had changed dramatically during the previous few years. BB himself admitted more than once he did not ever embrace/participate in her fitness passion. Additionally, a few remarks made by a relative of BB made it sound like there may have been some general resentment about the amount of time MB dedicated to fitness: "she didn't have time for anything else" - a quote from one letter. Then in another article she says she won't even speak the name (Camp Gladiator) "I guess because it took so much of her life, and then it took her life."
Obviously an absent wife/mother is not a motive for murder. But to me, it paints a picture of general resentment and dissatisfaction in the relationship. A change in roles. A hint at unfulfilled expectations. Perhaps a power shift in the marriage. The dutiful wife and mother finds new strength and success outside the home. Perhaps her flirtations or attentions have been discovered. The entire situation simmers and festers. Sympathetic ears turn into angry allies and stir the coals. The resentment builds. The rage. Some final straw pushes the situation over the edge, and plans start to materialize.
BB seems to be a soft-spoken man with nothing but love and admiration for his wife. But to my eyes, there is an undercurrent of disconnect in his words (and the words of his family members), that hints at something out of place in the family dynamics.
Again, not necessarily a motive, but some things to consider. There could be much more going on with BB, but I'll only speculate on published quotes that are verified.