I have some different thoughts on this, GB and AM were very close, he was AM's family, they worked together, and had that relationship for years. It wasn't GB that AM killed, it was JB, what if the reason they were moving to Arizona was because JB wanted to get them away from AM?
AB was 14, you read what her gf said about her and friends being creeped out by AM, always being at the park with the girls. AB wasn't young anymore and easily influenced by AM, her relationship to him could have changed. She probably was seeing him more as her friends were. GB posting on JB's fb about coming back home, we can work this out, could have been a rift about AM in their lives. He was going w/GB to take things to their new home in Arizona, maybe he wanted to move there, also. Could be why GB isn't speaking out, if this is the case, he could be feeling guilty that it all led to this horrible situation. This seemed like an angry revenge murder, because it was the first thing AM did.