When it comes to determining the difference between simply killing, or committing murder, for instance. But the motivation for committing murder is irrelevant. Cersei's willingness to murder Robert whether to protect her children or because of spite or because she wants to rule the kingdom herself does not alter the right or wrong of the action. It is still murder and even the assertion that he would certainly have killed her children does not justify her actions in response (Ned certainly offered her a morally acceptable solution) or Ned's willingness to give her a head start. However wrong Robert might be to kill innocent children, that does not justify using immoral means to cover up immoral deeds. It is Cersei's fault the children are in danger, since she was the one who placed them in that position. She does not get a pass because she is in a tight spot because of her previous crimes. By that standard, any criminal who kills a policeman to abet his escape is justified, since the police officer intends to lock him up, and possibly even put him in danger of execution, or being shanked in the yard.