Most violent games do warn before you play, or have an option to take off blood as plokk says. But for legal issues almost all games have warnings as you load or on the main menu.
What about in Punk O Matic where you have excessive swearing in just about every part of the game? There's no warning on that. There's only few games that don't warn. Most of them do warn however.
Most games will warn you in-game beforehand if it is particularly violent. Otherwise, people younger than 13 years old should not be playing on this site. And I would imagine that most people 13 or older either would not be as bothered by it, or could read the description.
