In Viking Law, men were allowed to simply kill other men, however men (and women) were not allowed to kill women or children in cold blood (unless they were holding a weapon).
Killing women or children in cold blood was punishable by death unless the killer was Royalty, in that case temporary exile was used.
Women who managed to kill a man, would become a Shieldmaiden, therefor becoming equal to men in law, meaning other men could now kill the Shieldmaiden in cold blood.
The first kill for a Shieldmaiden could be made in battle, or just some random dude in town. That random dude's family probably wouldn't be happy though, so that Shieldmaiden probably would only be one for a minute or so.
When it comes to gameplay, it makes no sense that I can't slaughter men needlessly while on Raids, they are fair game according to Viking law.
I would understand if I was not allowed to kill men while just roaming, but on Raids?
Come on man lol, let me have that GTA Viking experience.
On a side note, I feel like Ubisoft had the misunderstanding that all women in Viking culture were fighters, they were not.
Women were often treated like property until they became a Shieldmaiden.