It's not only a human notion. Two species of bugs have been shown to wage war, and in the case of the slave driver ants, carry off the dead colony's young to become their workforce.
Vikings were basically just raiding parties. They made people miserable and very dead. It's alright to enjoy fictional stuff. As long as you remember the actual historical vikings were as scummy and cruel as the rest of ye olde humanity.
I honestly don't believe in sin. I believe in ethical behavior. Sins were made up to oppressively micromanage people's lives to impossible standards. Ethical behavior allows a good balance between keeping our society running smoother, while allowing for reasonable amounts of personal freedom...