Aslaug's always had a hunger for power and the finest things. Her only redeeming quality I guess would be her love for her children? (much like Cersei in GoT) but thats it. What else about Aslaug is captivating or worth rooting for? Even Siggy (who was sketchy most of season 1) had qualities in her that I grew to love even though she wanted to get power back. Aslaug was never written to be liked imo, I dont think I know anyone who likes her.

I personally have never liked her since her introduction and I'm not surprised that by season 4 she wants total power and (its hinted) she wants Bjorn out of the picture or dead and doesn't care about Ragnar. So my guess is shes gonna do something thats a good enough reason for Lagertha to rise against her.

Also, if I were Lagertha, I'd probably hold a grudge too, you can't blame her for that. Sure Ragnar cheated on her, but Aslaug knew about Lagertha and did it anyway, it takes two to tango. Lagertha was the bigger person and left, but that doesn't mean those feelings of anger disappeared, she was just mature enough to put it away. A lot has changed since season 1 and 2.

This L vs A could be fan service, but I think they will give us a good reason for it. But as for me, I've wanted Aslaug dead since her introduction and even more so when she agreed that Athelstans death wasn't in the wrong. She just isn't likeable and I think its very much in character of her becoming a manipulate/evil queen.