The Fetus as a Final Boss is used primarily because of the inherent juxtapositions between the fetal form of a child (and everything it represents) set against the story arc and general tropes of the video game formula.
The End of the Game Vs. The Beginning of Life
- The Final Boss is traditionally the last gameplay encounter players will have in a video game; it's a definitive end point for a game, the end of the story arc and the last, most interesting encounter in the game. The Fetus as a Final Boss plays off of that expectation by presenting a creature that conjures in game players the image of new life. By using a Fetal Form for a Final Boss, the game designers juxtapose the finality of the end of the game with the new beginnings associated with birth (i.e the End Vs. the Beginning).
The Giant Final Boss Vs. the Baby.
- Players have been instilled with the expectation that the final boss of a video game will be a giant, monstrous creature. Final Bosses are towering opponents that, at times, barely resemble a human form (or what has been established by the game as "human" or "normal"). By contrast, the fetus conjures up images of weakness and fragility, of a creature that wasn't allowed the opportunity to grow up.
- The Fetus as a Final Boss changes the dynamic of the final encounter: a traditional video game end boss makes the player feel like they have slain evil and performed work that was "good" or "helpful" or "righteous". By contrast, when the final boss is a Fetal Form, regardless of the armaments and tactics it will use against the player, the player takes on the role of an aggressor, destroying a creature that is too weak to protect itself.
- The Fetal Form is also associated with innocence, playing off of the belief that babies are not born evil, but that they become evil through the environment they are nurtured in. By making the final boss an "innocent baby", the player is forced to deal with more complicated questions about the nature of the final battle. It also forces the player to reexamine whether they are the protagonist and antagonist in a fight.
The Termination of the Fetus and Parallels to Abortion
- In the majority of video games, the player's objective in the fight against the Final Boss is to kill the creature, thereby ending the threat against the world/saving the day/ending the game. However, when the designers make the final opponent in the game a fetus, they (consciously or not) draw parallels to abortion (the process in which a female ends her pregnancy by removing the fetus from the womb, ending the gestation of the egg and preventing the birth of offspring).
- By having the player kill an enemy that looks like a fetus, they are essentially forced to perform an abortion. It's an action that is specifically designed to be uncomfortable, playing off of the moral and political dilemmas of the abortion process.