Herfjötur stopped Erik from intruding on Lagertha's time with her sons or even harassing Lagertha. Erik hated Herfjötur and she knew it.
"Erik The Berserker, I am surprised you are not chained up beside King Askild like the dog you are," Herfjötur insulted Erik, she was not afraid to anger him despite his animosity towards her because she knew Erik could not kill her not because it would anger King Askild, but simply because he could not.
"Why you….!" Erik wanted to strike her so badly but he could not bring himself to do so, he kept his rage controlled because he was well aware that things could spiral out of control if he did act recklessly.
"Erik, do not speak to me like you could take me on in battle, or do I need to remind you about what happened the last time you tried?" Herfjötur asked him in a cold-blooded tone as she walked up to Erik, she was mere inches from his face as she looked him in the eyes.