The most violent Anime series I have seen till date are:
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Hellsing Ultimate (That scene where Seras' mother got killed and then rapped right in front of her when she was still a kid.. was crude and traumatizing as hell... seriously I have yet to see another series that would take things this far)
This video gives u a pretty good idea of what I am talking about
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Gantz
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Higurashi no koro ni? lol this one was funny too
There was this other one with the pink haired chicks with telepathic powers where people got hacked to pieces, that one was pretty violent too but I can't remember what it was called.
EDIT.. just found the name of that ^ anime a few posts above....
Elfen Lied, yeah that one was pretty shocking at the time