I think Brandon was poisoned, as in he died because his food was dosed with fentanyl.
I've mentioned his lack of mentioned history of drug abuse, but there is a hinted ... sensitivity to even a hint of someone not supposed to be near food. Like someone pretending to be a chef. Not even the guy who Brandon thought was the imposter of his beloved baby brother got it that bad. Mark would have had an easy way to get the drugs. all it took was one nurse walking away, or he pocketed a pill and told a nurse he dropped one and thats that.
Olivia tried to get away. She probably didn't see the water until it was over her head. It was at night, dark, and she was suffering from blood loss. not counting the fact that one of Mark's precious starlings killed her, or the fact that he ignored her call, why were there no lights or anything signaling the fact that the lake was right there?
i think it's more telling that the zombies were able to fuse since one killed 3 parties involved but they really decided that "fuck that guy" was too important, or they decided that Mark was the cause of all of their suffering. probably true; if Brandon died anywhere else, they would not be there.