I remember reading about a slightly different version of the bear hunt ritual.
The hunt would take place at winter, when the bear was hibernating. The hunting party would slay the bear, and although they would consume the meat, the skeleton would not be harmed (even though bone marrow are one of the most nutritionally dense foods available in a snowy winter, so this was a great sacrifice). The skeleton would then be cleaned and brought back to the burrow where the bear had hibernated, and then put together again (the vertebrae placed in order on the ground, every bone in its right place, ect.).
This ritual was to make the bear 'come alive' again, so that it could be hunted anew. My guess is that Lalli is doing something similar - cleansing it from the troll stain so that its soul can be healthy again and move on or something.