Dark anime

I watched Denpa Teki na Kanojo a while back now and am trying to find similar dark / twisted / horrific anime, I was suggested Elfen Lied so watching that now...

Any suggestions?

Depends what you mean by dark.

You should definitely see Devilman OVA's and Kite.

If you're looking for gore, there's Apocalypse Zero and Genocyber.

The Kara no Kyoukai movies have a lot of horror and dark elements, especially the third ova. You might also like Mnemosyne.

Shugurui is a gruesome samurai show. The mangaka for Shigurui is also the one who made Apocalypse Zero.