70% of today's anime series are filled with violence and somehow it's become a standard factor, even those most "innocent and cutest" very often have some amount of violence. Anywayy there is a fairly number of violent anime series that I've seen but I'll just mention few of them:
Gantz - anime full of meaningless violence accompanied with lot of hentai scenes with mixture of bizarre and disappointing story.
Elfen Lied - This anime starts with scene of naked girl casually walking and killing large amount of soldiers leaving every room fresh painted with lots of blood, the rest of anime isn't any different although there is less blood.
Fist of the North Star (1986) - some guy that sounds pretty much like Bruce Lee kills his opponents in rather unusual way - head exploding everywhere. And I must add that the story has no sense at all, sucks.
Kenshin OVA episodes - Kenshin past is shown much more detailed so are his killing and even tho there are only four episodes there are a decent amount of violence.