Didn't the Seer tell him Paris would be conquered by the dead? So he knew that many would die and might as well get revenge on Floki for killing Athelstan. IMO

:(

Yes that's true, but it got rid of the annoying kings son and the other men that supported his usurper. Now he can freely show Lagethra how much he's always loved her and feel that he is her equal.

Didn't the Seer tell Ragnar that Paris would be conquered by the dead? So with Ragnars' knowledge of that, it made sense to me that he would set Floki up to fail (knowing many would die) and it's a win win for Ragnar. Floki is to blame,thus he has some revenge.