Whether you've watched the anime, read the manga or played the games, any Yu-Gi-Oh! fan is familiar with the focal game of Duel Monsters. Pitched as a children's card game but with a depth and complexity that makes even longtime fans pine for a comprehensive guidebook, Duel Monsters has players using monsters, spells and traps to reduce their opponent's life points to zero. If a player is particularly unlucky, this ends with a one-way ticket to the Shadow Realm (in the dub) or the playing of a Penalty Game.
Despite being billed as a game for kids, Duel Monsters was the ancient Shadow Games made real in modern times, at least according to the original series. While the later spinoffs treated the game with varying degrees of seriousness -- from warranting a school focused on the game to being able to duel your way out of getting arrested -- the first series established that the monsters themselves were, in reality, the monsters inside all of us, the ka that could be forcibly extracted from criminals (and sometimes innocents) or willingly brought forth by the protagonists. The death of the ka meant the death of the duelist, and these life-and-death stakes continued in the modern setting the characters most frequently found themselves in.
In the anime-exclusive "Waking the Dragons" season, Yugi and the Pharaoh learn of the existence of the Duel Spirit Realm, an alternate dimension exclusively populated by the Duel Monsters themselves. Later spinoffs such as Yu-Gi-Oh! GX and Yu-Gi-Oh! 5Ds would continue to make use of this realm, and even make extensive trips (or, in the case of GX's Bastion Misawa, decide to live there full-time with the Amazoness card of his dreams). This establishes that the Duel Monster cards are indeed alive and sentient, which raises some major ethical questions regarding their use and suggests a sort of creeping horror regarding the daily life of such a monster.
After all, one of the major mechanics of the game is sacrificing weaker monsters to bring out stronger ones -- and this isn't even getting into the concept of fusion summoning, in which two or more monsters are fused to make a new one. Later series are a bit nicer in their depictions of these summons, but early installments were not, showing the monsters disintegrating or exploding. In one memorable instance, having two disparate monsters being force-fusioned resulted in them slowly melting in agony.
These are living creatures whose entire existence is at the mercy of the duelists, who may or may not treat them with anything regarding care. Take, for example, Seto Kaiba ripping Grandpa Muto's prize Blue Eyes White Dragon in half, or the sheer callousness with which the Rare Hunters treat their cards. Some are capable of fighting back, such as the Egyptian god monsters, but most must hope that their owner is benevolent, or at the least merciful.
And even before the concept of these monsters being real beings is made manifest in later story arcs, the original series shows this clearly, with the monsters having realistic reactions to the actions taking place on the field--Kuriboh's responses are often played for laughs, but in many cases you can see the monsters in agony upon being hit, and during the Arkana duel in Battle City, Arkana's Dark Magician is horrified at the notion that his master would so callously destroy him for a quick gain. Since they can be summoned back from the graveyard or used in later duels, death is not a release for a suffering Duel Monster.
The more you consider the concept of Duel Monsters as living things, the more terrifying their very existence becomes. Low-powered monsters must hope for buffs or come to terms with being fodder for traps or larger monsters. The sight of face down cards elicits fear. Chumley Huffington from the GX series having a heart-to-heart with one of the cards he intends to swap out of his deck is initially played for laughs, but how many cards didn't even get that courtesy?
But there's a heartwarming aspect when you consider the monsters' relationships with the main characters, especially in GX onward where the Duel Monster spirits directly interact with the characters. These genuine friendships give a different spin to the "Heart of the Cards." The cards themselves want to see their duelists win and are fully capable of responding to their needs. The existence of a Duel Monster is a harsh one, but sometimes it can be rewarding.
