The ESRB has put forth an official decree: Red Dead Redemption is rated M because of drugs, violence, nudity and harsh language. There’s also excessive blood.
Read the official ESRB statement below. (Some of the description might be considered spoilers for some.)
Red Dead Redemption
Platform: PlayStation 3, Xbox 360
Content descriptors: Blood, Intense Violence, Nudity, Strong Language, Strong Sexual Content, Use of Drugs
This is an "open-world" action-adventure game in which players assume the role of John Marston, a reformed criminal on a mission to capture the remnants of his old outlaw gang. Players roam Wild West environments and perform various missions for criminal figures and law enforcement: destroying bridges, helping individuals in distress, delivering packages of drugs (e.g., opium), capturing outlaws, and eliminating rival figures. Players use shotguns, pistols, and rifles to kill enemies in realistic gun battles. Players can engage in melee-style hand-to-hand combat and trigger slow-motion effects by shooting enemies at close-range or by targeted shooting (i.e., aiming for body parts). Blood often spurts out of characters when shot; head shots and knife attacks result in much larger sprays. In one sequence, a bloodied and mutilated corpse can be seen hanging from the rafters of a barn. Players have the ability to shoot pedestrians in the game; however, a "Wanted Level"-penalty system discourages these acts by triggering a law enforcement response. The game contains a scene in which two characters have sex on a table; there is a brief instance of female nudity. One sequence depicts a man injecting himself with a syringe filled with cocaine. Language such as "f**k," "c*nt," and ‘sh*t" can be heard in the dialogue.