Yugioh trading cards

I bought my son some of the Yugioh trading cards that "all the other kids had" I thought they would be pretty harmless. While these are just trading card, I thought the the characters were rather dark and disturbing to look at. For small kids I wouldn't like this at all, but even for my 9 year old I am not a fan and wish I never bought them. For instance the character "Doomsday Horror" he is "sent to the graveyard" and is considered one of the "dark monsters". This is not something I want to expose my kids to. Some of them are very creepy.