4Kids made him into a prick that never shut's up about justice and a Literal "heart of the cards." In the 4Kids version of the anime, Yami no Yugi spends most of the duels Talking!
In the Japanese version of the anime Yami no Yugi still doesn't seem to know when to shut up. However unlike in the 4kids version, Yami no Yugi doesn't shut up because he's busy taunting his opponent while planning his next move, and Not mindlessly going on about justice and what-not. He also does mention "heart of the cards" a lot, but it's a metaphor. (Trust in yourself and in your abilities etc.)
Now if you read the manga or watch Season 0. that is where Yami no Yugi gets a little scary. Here you don't see the 4Kids prick, or the Japanese anime Yami no Yugi. Instead you get a slightly sadistic spirit that likes to play penalty games on his victim, where said victim usually ends up suffering severe torture of some kind, and in the worst cases death. In the manga and Season 0, Yami no Yugi forces Yugi into unconsciousness so the spirit can do as it pleases.
Random Person: They're freaking pieces of cardboard you prick!
(I'm just making up the cards. I don't remember if they exist or not)
Regular Japanese Yami no Yugi (talking to opponent): You're a coward you know that? I bet you would never dare play if you couldn't rely on that Dark summon card of yours. (Thinking: I should use Revealing Sword and then use an anti-defense card.)
Random Person: Yami no Yugi actually has a brain!
Manga/Season 0 Yami no Yugi: Penalty game! *Laughs as unfortunate victim suffers severe torture.*
Random Person: I don't know what's scariest, Yami no Yugi, the fact that Yugi is completely oblivious to Yami no Yugi's existence, or that nobody has noticed anything strange about Yugi and sent him to a shrink.