If evidence comes up in the sequel that Brandon did have a drug use problem I will back down on that point but there’s no evidence of that now and even what else is documented and how well everything is documented, I’m considering that the absence of evidence is the evidence of absence.
That being said, Marks actions caused the death of a lot of people regardless of whether or not he weaponized the starling he created.
Plus fentanyl isn’t like today. It was harder to get a hold of in the eighties and Mark having a hospitalized, sick wife means that he had a better chance of getting it as opposed to a guy on an island where everyone is packed together.
With how desperate he was to get Rex back and how perfect the condition the body was (which is also strange, prolonged drug use causes damage to most systems in the body. Brandon just dropped dead and he was good to go after his system was flushed.)
Maybe Mark didn’t do; it there was another guy that wanted the role of Bucky but Mark had a more reasonable means to access the cause of death and Bucky going after Gary as brutally as he did makes a bit more sense if he died via food tampering.
Brandon thinks he’s Bucky. He should have gone after the guy he thought was impersonating his brother the most violently. We see in hotel investigations that his brother is his priority. Bucky doesn’t hate the goose. There’s be no reason to snap him like a Kit Kat and fry him. There were cleaner and faster ways to do it. Knives.
And there is a reason for Mark to do it this way if he did. He needed a clean control to start with the starling project and killing Brandon like that would offer that- because Mark was using them to figure out how to bring back Rex. Killing Brandon like that would offer a body with the fewest work that needed to be redone on it before they got to work.
Maybe he did do it, maybe he didn’t but the situation doesn’t look great for him.