Should I Accuse Galinn?

There's reason to suspect each of them, but only one is the actual traitor. If you don't care about becoming Sherlock Holmes for a short while, then you should know the traitor is Galinn. Accusing Galinn results in a cutscene where Soma slits his throat, but she is ultimately thankful for you finding the traitor.

What happens if you accuse the wrong person Valhalla?

If you choose Soma's traitor incorrectly, it'll affect Soma's standing with you, and Birna will not join your clan. Galinn will also later admit his guilt and try to murder either Birna or Lif. If you choose Galinn, Birna will eventually join your clan and you will be able to take her on raids.

What happens if you dont choose Galinn?

If you didn't kill Galinn, he will later attempt to kill either Burna or Lif (depending on who is alive), and this will result in you having to fight him.