"Tales of the Black Freighter" was a backup cartoon in the original "Watchmen" comics. And yes, it's adult. I'd also say the Hellboy comics published by Dark Horse are adult- they play with disturbing images (think of the "tooth fairies" of the second movie- Kinda cute, but they eat you to get at the calbium in your bones and teeth. The animated Hellboys were made more "Kid-friendly", but there are still some disturbing stuff in there. So is "Heavy Metal", the Movie, much of which is about the havoc a strange alien globe wreaks on anyone who comes into contact with it. Animation was by Moebius, I think.
The 70's were responsible for some very weird and strange animated movies/cartoons. And the Japanese do plenty of "adult stuff" in cartoons, like Crayon Shin-chan, which even though it's about a toddler, deals with more "adult" content. Or look at Stewie Griffin of "Family Guy", a baby who has dreams and fantasies of romancing his own mother, or killing her off. That's pretty adult/mature content, right there. And then there was Aeon Flux, which first appeared in "Liquid Television" on MTV... she died at the end of every episode. They had some other pretty disturbing animations- I recall one about two racers who were so intent on winning a race that they killed each other off and the entire stadium went up in flames, with the racers' skeletons driving around the track of flaming skeletons forever in Hell (I think it was Hell, there was no dialogue to tell you what was happening).
To go on to Anime, take a look at the series Vampire Princess Miyu. In the original, long animations, she is depicted as far more cruel, while the later series that had het go to a normal High School is also very adult. Some of the episodes actually made me cry, and I am a 44 year old woman, less disposed to cry than most.
People think cartoons can't be mature because a great majority of Western cartoons *are* for kids. But it's never been only *all* just for kids. But unless you are into animation, that is all most adults ever see. I mean, even cartoons "for kids" can be sick, twisted and downright disturbing. Just look at "Courage the Cowardly Dog". Some of those episodes are downright Nightmare Fuel.