Is Lif the Traitor?

The traitor is Lif. The traitor is Galinn.

Who is the traitor LIF or Galinn?

If you'd rather just know and be done with it, the real traitor is Galinn. Soma will react by executing him and Birna will join you and the Raven Clan. If you're curious about how that conclusion is reached, talking to each of the suspects will reveal that Lif had some yellow paint stolen.

Who is the traitor in Valhalla?

The traitor is Galinn. He wanted Soma out of his way to fulfill the true fate he has seen in his deluded visions. He disagrees with her ways. He was far away from other people in the village when the attack happened -- without an alibi -- and he is quick to blame Lif, and his longship is painted yellow.