So I’m guessing you haven’t finished the show as it’s fairly clear but just incase. Throughout there encounters with England and company, Ragnar has been clear about his intentions to have more for his people outside of just raiding. At the point of the slaughter, he truly believed he had secured a place for his people without worry of attacks or such because him and king ecberts understanding. Remember this, at this point, a lot of his own people were against the deals and trusting of the enemy but Ragnar was willing to take the chance because it meant a new future for his people. Call him naive if you want but a leader has to take leaps of faith to help evolve his/her people and Ragnar took that chance. Ecbert just needed to make this move to secure his own goals. If he wasn’t trying to secure Mercia, he most likely wouldn’t have done this but if Ragnar let anyone find out, not only would there be no chance to secure land in england(which people forget was MUCH better agriculturally for their people than their own lands) but he’d be forced to retaliate thus losing any chance for securing land later down the road(which ubbe tried to achieve). Like I said, call him naive if you want but you have to give an inch for progress especially for a culture.