If Electra is emotion or duty-driven, Chrysothemis is reason-driven. It's not that she's immoral for taking the side of her father's murderers. Rather, she's too pragmatic to commit herself to what she considers to be a futile course of action. Time and time again, she begs her sister to be more reasonable. But Electra will not be met with reason; she is set on a particular course of action and will not be deterred. Electra responds to Chrysothemis's pleas with hostility and accusations.
We have to wonder what attitude the play takes toward this character; is it an understandable course of action that Chrysothemis has chosen? The Chorus, made up of the women of the town, does side with Electra's sister for much of the play. That's good evidence for the argument that the play condones her behavior. But what of Electra's conclusion? Chrysothemis defends her decision on the grounds that Electra's actions are futile – but the play's ending shows that her actions were not futile at all. Through determination and will, Electra brought about the justice she so vehemently sought. When you look at it this way, it's tempting to judge Chrysothemis for not having the courage her sister did. This is not the only way to view the play, however. Feel free to be creative in your reading of Chrysothemis's actions.