Oh my god, yes, someone asked about Japanese stuff, including names! I WAS BORN FOR THIS!
Let's see. Shuurajou means "A scene of carnage" and references the battle ground where the Asura and Sakra war against each other. "Koroshiya" is just a word for professional killers. You could call it Yamata no Orochi after the famous "8 headed, 8 tailed serpent (dragon)" that... tricked the sun goddess, as I recall, and you could make it a gang that started with or consists of 8 families.
Continuing that thought, I'd go with a family theme personally, kind of like the Yakuza (Japanese mafia), and name the ranks accordingly. Like, Aniki is a gangster type title for an "older brother" or "upperclassman" who you look up to and who protects you. I can tell you more if you care.
A good name for a protagonist and antagonist, huh? Well, for boy protagonists: Hikari / Hikaru means "Light" or "Radiance", Akihiko means "bright prince" Akio means "Glorious Hero", Daichi means "Great wisdom" Hideo means "splendid man" Hiro means "generous," Kenshin means "modest truth." Isamu means "courage."
For girl protagonists: Mikomi means "Hope" or "A chance", Akiko means "Bright child", Atsuko means "Kind child." Chika means "scattered flower" or "fine wisdom." Emi / Megumi means "Blessing" or "favor" Etsuko means "Child of joy," Haruko means "child of sunlight." Hotaru means "Firefly." Hoshi means "Star." Kotone means "sound of a harp" Makoto means "sincerity."
For boy antagonists, Hachiro means "eighth son." Actually, there are a -lot- of names like that, first son, second son, third, fourth, fifth, so on. You'd easily be able to name all of the leaders of the eight families if you went with that theme. Akumu means "Nightmare" and Yami means "Darkness, black market, the dark," Kage means "shadow / shade" Kurai means "dark" Oni means "Ogre" Hotaru means "despair" and Akuhei straight up means "evil" Yakunan means "misfortune." Neikan means "wicked, trecherous"
For girl antagonists, Chi means "Blood," Chiyoye means "Everlasting pain," Sayomi means "night-born," Rin means "Cold" and Tora means "tiger" Nikusui is the name of a "monster that appears as a young woman and sucks all of the flesh off of its victim's body." Kiyohime is "a woman who transformed into a serpent-demon out of the rage of unrequited love" and Kira means "killer"
There were female army generals, female ninjas ... and yeah, just re-checked, there were female samurai, too. They typically used a weapon called a naginata, which is basically a sword on a stick. Exhibit A, wikipedia "During the Edo Period, as the naginata became less useful for men on the battlefield, it became a symbol of the social status of women of the samurai class."
I can try and think of more good names if none of the above suit your tastes for a samurai name.