By Viking law, if you killed them first, it wasn't stealing. And theft from the living was considered a great sin.
Well sure... Put that way, murder would make mere theft irrelevant... to the murderers and thieves anyway.
Old Norse culture also treated murder a bit differently than we do. If one was convicted of murder, one was banished from the "civilized" Norse lands for some time. This lead to things like the Norse colonization of Iceland, Greenland, and Vinland (what folks call "Canada" today). After a while, they were usually let back into the "civilized" lands.