So now I am going to try and explain the Blackwater Massacre, the bloody and massive gunfight between the Blackwater Police Department, the Pinkerton Detective Agency, possibly Landon Ricketts and the infamous Van der Linde gang. The massacre was caused when the Pinkertons attacked the gang during a ferry job with the cold blooded murder of Heidi McCourt at the hands of Dutch van der Linde.
Before the massacre, the gang was doing fine, staying on the hills of the Northern Grizzlies following a fire in which Arthur lost his journal. On the road, they picked up two new members: Jenny Kirk, a young abandoned girl they meet on the roadside, and Micah Bell, a outlaw who saved Dutch's life after he offended some locals when trying to sell some gold they stole and he decided to offer Micah a position in the gang. Arthur mentioned that he heard a tale of a man who sounded like Josiah Trelawny, but he couldn't believe it, since they haven't seen him in many months but we will get back to that. The gang decided to camp outside Blackwater, a growing port in West Elizabeth. One day, Arthur and Hosea learned of a big lead, a real estate scam they decided to go after, but while they were doing that, Micah told Dutch of a ferry that was carrying loads of money and gold to the bank in Blackwater. After being convinced, Dutch decided to pursue it, despite Hosea and Arthur's protests of it not feeling right. Hosea and Arthur decided not to participate in the job, with Dutch and Micah planning the job instead. The job then was executed when the gang raided the ferry. Jenny is mentioned to have been shot and later died during the robbery so she would have a role as a distraction in the job. And now about the brutal murder of the young mother Heidi McCourt, killed by Dutch. I personally believe that she was a hostage held by Dutch so no one would try anything funny and force the others to hand the money and gold to the gang, but then the Pinkertons learned of what was going on and attacked the gang, with most likely Milton and Ross being a part as well, with the Blackwater Police later intervening as well with famous gunslinger Landon Ricketts also joining against the gang. When the shootout happened, Dutch accidentally killed McCourt in a panic in which the gang and most likely Milton witnessed, being disgusted but trying to continue fighting their way. Jenny was most likely shot while escaping with the others. When Hosea and Arthur was most likely done with their scheme or they had to stop their scheme, they learned of the gunfight and quickly headed to the shootout to help the others escape, this is where Arthur's horse Boadicea was most likely killed, possibly during their escape. They quickly head back to camp and get everyone to pack up and they escape into the mountains. Of course, Davey is shot in the gut but makes it with the others, Sean however is knocked out and then captured by bounty hunters. Mac was split off from the gang and tried to escape Blackwater, where he was caught up by Milton who decided to kill him. Jenny died during their escape, with Davey later passing away when the gang reached Colter. John was also shot in the shoulder during the massacre by what I would guess is most likely Landon Ricketts himself but it was only a flesh wound, as he would later get sent scouting.
Now about the Trelawny thing, the man who was described like Trelawny is most likely the Strange Man himself since the gang hasn't seen or heard about Trelawny for months before the massacre and he was most likely watching from a distance in Blackwater and probably the heist.
Now how the Pinkertons knew about the ferry, it was most likely I guess was a tip off from no other than Micah himself. There is evidence that he had a newspaper scrap of the massacre and a wanted poster of Dutch and most likely wanted the Blackwater money and tried to have Dutch turned in.
Anyways, that was my theory/thoughts on the Blackwater Massacre. Let me know if there is anything else I should have discussed about.