The Sherman children seem to be very afraid of further violence towards them, or their families. This is one reason they gave for not wanting the terms of the will, or wills revealed to the public. The Judge at one point said he thought there might be a real threat of danger, and agreed to seal certain documents. I think that was Barry's will, and maybe Honey's if they did find one.
I agree with you that money is very likely a big part of the motive for murder somehow, but it may be indirect, in that the Shermans did have a great deal of power. Maybe they were somehow using that power politically, like fund-raising for politicians or organizations, in a way that threatened the interests of other wealthy people, or even governments and business interests in other countries. I think the " rabbit hole " goes deep in this case, and very powerful people sent a message with the horrendous manner of death in the Sherman's own home, where they felt safe, then the grotesque display of the bodies. In my opinion, the Sherman children got that message, whatever it meant, and that is one reason they are afraid. I noticed that shortly after the death of Barry and Honey, Johnathan, the son, said publicly, and I think it was during his eulogy at the funeral, that he and his sisters intended to continue the charity work of their parents. Were the Shermans going to stop giving money to some organizations, or were their humanitarian and fund-raising efforts somehow politically/and/or/financially threatening to powerful groups of people, or individuals ? It is hard to say, but I think the motive is somehow connected to them antagonizing some powerful people or organization, and then refusing to stop what they were doing, after they were warned. Barry was known for being very stubborn, and arrogant. Maybe Honey saw her actions as them being determined to do good, in the face of opposition. The display of the bodies had to be a message of extreme hostility, hate, and a threat to the family, and others, but I do not know what it was, and we may never know.
IMO