You might have heard of the term “fridging,” which refers to a trope in comic books, TV, and movies wherein a character is killed to further the emotional arc of another character. It’s also sometimes known as “Women In Refrigerators,” because of an instance in the Green Lantern comics when a woman’s corpse was literally left in a refrigerator for her boyfriend to find. If you want another example: After Gwen Stacy plummets to her death in Spider-Man (both in the comics and the movie), Peter Parker is so heartbroken, it motivates his heroism forever after.
The character who ends up dead is, most often, female; and the one whose story is pushed forward by their grief is, more often than not, male. In Wonder Woman, though, it’s Steve who dies — in his own heroic moment, sure, but he’s dead. Super dead. And his death is designed to deepen Diana’s dedication to her cause. It’s a symbol, and one that has more power because of the trope it’s subverting. Steve’s death really hammers the point home: This is Diana’s story, and any man who plays a role in it — no matter how great he might be — is not going to outlast our girl.