Admiral William Stenz (David Straitharn) was supposed to die in Godzilla: King of the Monsters, and here's how it would've happened. The admiral had only a minor role in King of the Monsters but was much more important in the first MonsterVerse movie, Godzilla, when he was put in charge of the military's effort to kill the M.U.T.O.s. Stenz became determined to find a way to destroy both the M.U.T.O.s, and Godzilla. Throughout the movie, Stenz listened to the advice of Dr. Serizawa (Ken Watanabe), who urged the admiral to "let them fight".
In Godzilla: King of the Monsters, Stenz was present at the hearing presided over by Congress, when Monarch scientists made their case for keeping the Titans alive. He reappeared in Mexico for Godzilla's second battle with King Ghidorah; after both monsters were submerged in the ocean, Stenz communicated to Monarch through a monitor and explained that he was under orders to kill both Titans with the Oxygen Destroyer. Later, he briefed Monarch on Ghidorah's activities as the alpha, which was when Mark (Kyle Chandler) interrupted and pointed out that if Ghidorah could be defeated, the rest of the Titans would stop wreaking havoc. This was Stenz's final scene in the movie, but that wasn't originally the plan.
Godzilla: King of the Monsters director Mike Dougherty says that the original plan was for Stenz to die in the battle at Washington D.C., which was when the humans were trying to hold off Ghidorah and Rodan [via Twitter]. It was during this time that they were still waiting on Godzilla to return to action. A deleted scene from the film shows Admiral Stenz delivering a message to Colonel Diane Foster (Aisha Hinds), letting her know that his submarine took on too much water during the fighting. Stenz is cut off as his submarine is seemingly destroyed, presumably by Ghidorah. This was also touched on in the novelization, with Stenz's death being assumed but not confirmed.
Admiral Stenz's planned death means that Godzilla: King of the Monsters nearly killed all of the 2014 Godzilla's returning characters. Dr. Vivienne Graham (Sally Hawkins) died in Antarctica when she was eaten by King Ghidorah, and Dr. Serizawa sacrificed himself to save Godzilla in a scene that purposefully reversed Dr. Serizawa's death in the 1954 movie. Of the three, only Stenz made it out alive, and his survival opens the door for him to return in a future MonsterVerse installment. This may not happen anytime soon, though, because he isn't among Godzilla vs. Kong's confirmed returning characters.
It's likely for the best that the admiral's death was cut from King of the Monsters; he was a key character in Godzilla but was underutilized in the sequel. At least Dr. Serizawa and even Vivienne were able to appear in important scenes before being killed off. Also, Stenz's death wouldn't have packed a punch for any of the characters in the movie, as no one seemed to know him on a personal level. This, combined with his lack of screen time in Godzilla: King of the Monsters, would have made his death feel pointless.
