Just because a movie is animated doesn't mean it is for children. Animated films aimed at adults may be difficult to find at your local theatre if you live outside a city, but you can get them on home video. If you like a good action film, animation offers special effects that aren't often seen in live-action movies. Take, for instance, Ninja Scroll from 1993.
It revolves around a ninja named Jubei who is coerced into killing his entire clan of warriors. Jubei succeeds in slaying his fellow warriors, and he ends up as an assassin for hire. After some time, a demonic clan of ninja called ‘devils’ sets out on a mission to take over the Japanese government, and they do not hesitate to annihilate entire villages in the most horrific ways. Jubei has to save the government and the people, and the fights are incredibly detailed and violent.
So if you'd like to find something new and different to watch -without the kids- you'll want to check out a list of very violent animated films at Unreality.
And speaking of Ninja Scroll, creator Yoshiaki Kawajiri is no stranger to violent supernatural movies, with his earlier films Demon City Shinjuku and Wicked City.
Another ultraviolent movie/ova would be Kite. It's also hardcore hentai on the uncut version.
And if you include TV series, I think Elfen Lied would definitely be on the list. And the series Now and Then, Here and There should also be considered, for its depiction of child torture and rape. It's still one of my favorite anime series.
And these are just the ones I can recall off the top of my head.