If you liked the Ichi the Killer manga, the Anime-Planet community thinks you'd like:
Meet Eiji Bando, a deranged, hentai manga-reading psychopath with a penchant for gunning people down. As a hitman, he works for the yakuza, causing plenty of collateral damage wherever he goes. Whether he’s joking about violating his male peers or just blowing people to bits, Eiji Bando always has plenty of disturbing fun in the process!
Though Gunmania is way less explicit than Ichi (if you've only read Gunmania you may be wondering how this is possible - TRUST ME), both are extremely similar. Both involve a complete psychopath carrying out senseless murders. If you liked one, try out the other.