But she [i]is[/i] reasonable. It's not like she goes on a spree killing. By the time she gets to O-Ren she's killed the two people that raped her and no one else. Even with O-Ren, she only kills those who are ordered to kill her - and even then she doesn't kill all of them. She spares one and though many have lost limbs they aren't [i]dead[/i]. She even sends Sophie, the protege, to a hospital after getting the information she needed. Her actual kill count is significantly less, she doesn't even kill one of the people on her hit list because Elle got to him first. 

She expresses remorse and guilt when faced with the grief her actions cause on the innocent. She's not just some gleeful killing machine, she's not taking joy in her actions. When she went up against Vernita she offered a duel at midnight specifically because she didn't want Vernita's daughter to see it - which didn't go to plan because of Vernita's actions. She is still thoroughly human and shows her feminine side as well as her assassin side. She is not unstoppable, she's flawed, emotional, and still very much motherly and considerate. She's single minded, sure, but not at the expense of her entire being. She is vulnerable as well as hardened.