It's not that I think this is ooc for Helena, given her background and horrible life, but that the show has consistently shied away from actually dealing with Helena's dark side, because it's neat to have a ruthless killer at hand for the good guys when the show needs one. To wit: there's no point at which Sarah, or any other character, bothers to tell Helena that killing and maiming people is wrong. She's just not supposed to kill or maim anyone they care about. Also, at no point is Helena ever confronted by someone whose loved one she killed, in a hostile fashion. She killed Katya and several other Leda clones before the encounter with Sarah changed her, but in none of the seasons did a relation, friend or partner to any of these women show up to find out what happened to them, or to confront the one who did it. Because that might make the audience uncomfortable with Helena being completely embraced by the other clones as of the s2 finale. Just imagine, well, Cosima saying something like: "You know, I'm really sorry for what Tomas and his followers did to you, how they raised you. But you killed my friend Katya, whom I loved. We need to be allies because they're after us all, but I don't want to be friends because of this." Or, say, an (adult) child of the woman who carried Helena and Sarah to terms shows up, wants to know what became of their mother who risked all to warn Sarah, and is horrified to find out she was murdered and everyone is best friends with the murderer. The audience wouldn't like it because all sympathetic characters are supposed to be friends, and I get the basic impulse, but I'd still have appreciated it if the show had bothered at any point to give Helena something of an actual redemption arc, instead of bascially going "aw, she's such an adorable murder baby" with her.