I think it was the original Mortal Kombat. This isn't the sort of thing I remember, so I can't say for certain.
If it wasn't that (though something tells me it was), the next game I remember is Turok: Dinosaur Hunter, which is the absolute latest game that could have been my first experience with mature content. It wasn't very good, and I never could figure out how the hell to beat the game without any cheats. The sequel, however, was pretty fucking sweet from what I remember. Number 3, not so much.