It would be easy to try and pit Lagertha and Aslaug against one another. After all, their relation is founded on betrayal. However,Vikings resists TV's incessant, tired practice of making women enemies and instead unites these women through the common understanding that, at the end of the day, it's better for them to stick together than to tear each other apart.
Neither woman is completely innocent, but what they have in common is the fact that both often end up with the short end of the stick. Ragnar is a lovable character in himself, and hardly the most guilty man on the show, but when it comes to Lagertha and Aslaug he certainly builds up a decent amount of betrayal under his belt, ever demonstrating that his loyalty for the women in his life does not run as deep as that for men.
Lagertha: Didn't Let Her Come Raiding
Ragnar slighted Lagertha from the very start of the show. He always treats her nearly like an equal, but not quite. Lagertha is an established, capable fighter, and yet when Ragnar plans his pivotal trip to go exploring west, he does not invite his own partner.
Lagertha is furious about this but eventually agrees to stay home and tend to the farm.
Aslaug: Left Her While Pregnant
It takes two, as they say, and therefore Aslaug should have known that getting involved with Ragnar meant that she could end up pregnant with a married man's child.
However, on the flipside, Ragnar also should have considered that he was sleeping with a person who may see their interaction as more than a brief, one-time thing. Aslaug tells him she bears his child, and yet he leaves her anyway.
Lagertha: Cheated On Her
It was generally understood during the time that married men would go off and sleep with women other than their wives, but that didn't mean that the wives had to put up with it.
Women had (slightly) more rights in Viking society compared to other cultures of the time (and some today). Lagertha's union with Ragnar was not one that allowed going outside of the marriage.
Aslaug: Married Her Without Loving Her
When Aslaug comes to Kattegat, Ragnar makes clear to Lagertha that he does not love Aslaug. This is meant to comfort Lagertha, but also bodes badly for Ragnar's impending relationship with Aslaug, as well as his carelessness in sleeping with her knowing the possibility of her bearing his child and how this would affect his relationship with Lagertha.
Such a situation didn't allow for how Aslaug might feel--for all she knew, he was in a loveless marriage and she took his actions as affection for her.
Lagertha: Lied About Aslaug's Pregnancy
Aslaug shows up pregnant, and Ragnar insists to Lagertha that he didn't know about her.
He doesn't make clear if this means he didn't know she was coming, or if he didn't know she was pregnant--although the fact remains that he doesn't admit that he knew she was carrying his child all along, a sort of betrayal in itself.
Aslaug: Abandoned Her
Aslaug and Ragnar's relationship began, admittedly, on shaky ground. She could hardly expect that he would be a reliable husband considering that their union began as an act of him betraying his current wife.
Yet, it does feel cruel that as soon as they are together he goes off on several raids that take him away from his family for years at a time, leaving her essentially alone.
Lagertha: Tried To Have Two Wives
Ragnar sits at a table next to Lagertha and Aslaug and, feeling awkward about the predicament he's in, thinks it wise to suggest right there in front of both of them that perhaps they could be in a marriage altogether.
His actions here are a complete step away from the Ragnar who was known to be devoted to Lagertha and treated her like a respected partner.
Aslaug: Tried To Kill Ivar
Ivar is born disabled and Ragnar immediately insists that it would be cruel to allow him to grow up in such a way. He suggests that Ivar should be put out of his misery, but Aslaug is of a different mind.
She wants to love Ivar just as he is. While Aslaug is asleep, Ragnar takes Ivar from his bed and places him outside in the cold, where it seems he intended the Gods to decide for him--or for Ivar to simply freeze to death. And all this without Aslaug's consent.
Lagertha: Never Apologized
Lagertha and Ragnar re-unite and the first thing Ragnar does is make a fuss over seeing Bjorn again. This, one would think, should be followed by a meaningful apology, or at least a heartfelt receival, of Lagertha--but no.
Ragnar treats Lagertha as if she is merely an old acquaintance, and goes on to raid with her and exists side by side as if he never broke her heart and publicly humiliated her.
Aslaug: Left Her Holding The Throne For Him
One of the most entitled things Ragnar does in relation to Aslaug is to go off raiding for long period of time and then expect that Kattegat is waiting for him whenever he chooses to return.
Aslaug functions as the leader of Kattegat for what adds up to be years when considering his voyages all together, and yet as soon as he steps back on land it's assumed she is once again secondary, merely a wife there for bearing children.