Rollo is an asshole, but he is thinking the long game.
1) He does genuinely want the princess to like him and saw that she was pretty upset about the marriage, so he will be willing to let things progress more naturally rather than force it, which will help secure her as an ally. He never did have a problem with the ladies ;p
2) Over half the viking camp potentially betraying him was a huge risk. Now that he is married to the Princess and they still need him to help vs. Ragnar, it is of low risk to him to wipe out the Viking camp and remove that variable from the gameboard
3) With the Viking camp dead, unless another party passes on this information to Ragnar, they will have little intel on the ground and it will be much easier for them to walk into a trap/ambush