This part of the show Vikings really upsets me and was the "jumping the shark" of the show for me. I believe the purpose was to make a religious/symbolic statement in the show to emphasize Athelstans "turn" from Christianity while also giving a moment of time to allow Ecbert to save him.

Constantine banned the use of crucifixion in 337CE out of respect for Jesus thus ending it's use in Europe. Also according to Church tradition, when Saint Peter was to be crucified, he asked to be crucified upside down as he didn't feel worthy to die the same way as Jesus. Thus we can assume that there was a taboo of using crucifixion in Europe. The only post-Roman crucifixions were in 14th-15th Century Japan and it is believed it was introduced by Portuguese missionaries through the imagery of Jesus' crucifixion. These crucifixions were especially used against missionaries and Japanese converts by the Shogunate to discourage the spread of Christianity in Japan.

That said, a captured raider would have had their head chopped like the dude in the first episode except the dude in the first episode should not have been executed but outlawed (meaning they have no protection from the law so family members of the slain could kill the outlawed man without retribution from the law) or forced to pay a weregild. (a monetary fine imposed for a crime) Erik the Red's (father of Leif Eriksson) father was convicted of killing more than one person and he was only outlawed from Norway which is why he and Erik left Norway to settle down in Iceland.

I want to emphasize that you should really take the show Vikings as containing fact with a small grain of salt. To us reenactors and real historians the show is kind of a joke.

Thank you for such a wonderfully detailed answer! I didn't think crucifixion had ever been practiced within Christianity, but I didn't realize it was officially outlawed. That scene did jump out at me as being jarringly pointless. I'm still enjoying the show, but it's slowly shifting into "guilty pleasure" territory.