Here's how and why Alex Kamal dies in The Expanse season 5's finale. The plot armor of the Rocinante crew was holding up well after The Expanse season 4, with Holden, Naomi, Amos and Alex all still alive in both James S.A. Corey's novel series and the Amazon adaptation. In the summer of 2020, however, allegations was made against Cas Anvar, who portrays Alex. An investigation was conducted, and while the findings haven't been made public, an announcement ahead of season 5's premiere confirmed Alex would not be returning for the sixth and final season of The Expanse.
As a result, viewers were left wondering whether Alex would be hastily killed off, or whether he'd leave the Roci crew unceremoniously in the gap between seasons 5 and 6. The Expanse opted for option one. Alex spends the first chunk of The Expanse season 5 on Mars attempting to reconcile with his family, but winds up embroiled in Bobbie Draper's smuggling investigation. In the second half of the season, Alex and Bobbie set off in the Razorback and rendezvous with Holden in the Rocinante. Receiving a distress call from "Naomi," Alex takes the Razorback on a rescue mission, and it's this decision that leads to his death.
The Expanse's very first season lays out the mechanics of the show's fictionalized space flight. Thanks to the Epstein drive, ships can travel quickly from planet to planet, but when a pilot hits the accelerator or uses intense battle maneuvers (actions collectively known as "high-g"), the human body needs chemical support to prevent an unpleasant death. To this end, many of The Expanse's ships are rigged with special padded chairs that inject a cocktail of drugs into the passenger during high-g. The mixture contains amphetamines, adrenaline and blood-thinners, which are added specifically to decrease the risk of stroke. During high-intensity space flight, the pressure creates a crushing sensation that is liable to cause aneurysms and strokes after a prolonged period and without the "juice" injections. Even with the drugs, there's a limit to how much high-g humans can take, and this is why Alex dies in The Expanse season 5.
Audiences have perhaps come to take the high-g drug cocktail for granted - a convenient plot device that explains why space travel is very rarely lethal in The Expanse. But Alex is so desperate to reach Naomi after her escape from Marco Inaros, he burns the Razorback recklessly without easing the gas, putting himself and Bobbie at risk from the lottery of high-g. Bobbie survives; Alex does not. The pilot's death is foreshadowed earlier in the episode when Bobbie points out the pair are already "juiced to the gills." This is another way of saying that performing high-g burns is becoming increasingly risky.
The abruptness of Alex's death in The Expanse's season 5 finale suggests the episode had to be re-jigged after the Cas Anvar controversy began. With the still image of his body and a seemingly dubbed-over line, that certainly appears to be the case. Since characters survive high-g maneuvers 99% of the time, it might feel like a cop-out to have a major character succumb to pure bad luck - especially a relatively young and fit Martian pilot. For any other main character, death by high-g stroke could be considered a weak ending, but given the real life circumstances producers found themselves in, the awkwardness of Alex's exit can be forgiven.
With that said, Alex Kamal's departure from The Expanse is partially derived from the original books. In Babylon's Ashes, Fred Johnson is travelling with the Rocinante, attempting to coordinate his forces in the ongoing war against Marco Inaros' Free Navy. The Roci is ambushed by Marco's Pella, and the high-g battle proves too much for Fred, who suffers a stroke and dies. The circumstances might be different, with the Razorback rescuing Naomi Nagata instead of fighting enemies, but the concept of dying from a stroke in the process of stopping Marco Inaros remains true in both cases.