Currently, there is 18 families out of the total 19 families residing in Holy Land, Mariejois. With one family being excluded, it being Nefertari's family. So i was thinking. If there were 20 "World Noble" families at the start of Void Country, with 1 family acting as a "traitor" of sorts since they didn't act like the World Nobles in question and refused to move into Mariejois, there should be 20 "D" families as well before they were kicked out by the World Nobles. After they were kicked out, the D families went their own way & and has been waiting for the chance to strike back and claim Mariejois back. So here are the list of D families we know so far that exists in One Piece...
- Monkey Family
- Trafalgar Family
- Gol Family
- Portgas Family
- Jaguar Family
....And 14 more D families who are not yet revealed in the story. Maybe their blood died down or something.
As for the Marshall Family, i believe they assisted in the taking down of the "D" kingdom back then. They were regarded as "Traitors", with the World Nobles guaranteeing them that they will reserve a position in Mariejois for the Marshall Family. Of course, being the type of people they are, the World Nobles sort of went back on their words and betrayed the Marshall Family when the entire "D" kingdom was defeated.
I believe Blackbeard had a fcuked up past. His childhood was ruined (SBS picture). So, he wanted to destroy the world, including everything the World Nobles owned. Therefore he decided to do it, no matter how fked up the method was, Even at the cost of lives who he deemed as "Not needed".