Heh, a Mathematicians answer
Don't recognise that reference. What happened? Unless off topic stuff is a nono here...
Short version: Kenshin was an assassin in 1860s Kyoto, but decided to quit when he met a girl. Girl turned out to be sent by a rival faction to off him, but she fell for Kenshin. She went back to her group to quit, planning to run off and live happily with him. They kill her for failing her mission and trying to leave.
Kenshin is
slightly upset.
He buries her, burns down the house she and he were going to live in, storms their Kyoto headquarters alone, and slaughters
everyone inside. Guards, bosses, their own assassins, everyone. When he's done, standing ankle-deep in a lake of blood, surrounded by bodies, he throws down his sword, swears he's never going to take another life again, and plans to spend the rest of his life atoning. But every other assassin in the country just hears 'Himura Kenshin assassinated everyone in a whole gang in one night! He's the greatest assassin in the world! (The
Hitokiri Battousai) Whoever takes him down can claim that title!'
And to keep this germane to the SAO side, I can see Laughing Coffin hearing about Titan's Hand being wiped out, and deciding Kirito is just the kind of guy they want working for them. And I'm honestly not sure how Kirito would react to that. He might be depressed enough to decide that's for him...
But that's all hypothetical, so who knows?