The decline of the Viking Empire can be attributed to a number of factors. Christianity, disease, in-fighting and a new found degree of comfort and contentment with their lives in new settlements.
My first factor of the Vikings decline I would like to bring up is Christianity. The Vikings were very quick to adapt to Christianity, as they believed in many gods so it was no problem to accept the Christian god along with their own. They came into contact with Christianity through their raids as they use to raid churches because they were both wealthy but poorly defended and were found to be an easy target. The Viking Age also saw a gradual conversion in Scandinavia itself, as Anglo-Saxon and German missionaries arrived to convert the pagans. By the mid-11th century, Christianity was well established in Denmark and most of Norway. It wasn't until the mid-12th century that Christianity became established in Sweden.
My second factor would be about disease. Tradition has it that a disease called Dupuytren's Disease (DD) was a disease that originated from the Vikings who were thought to have spread it throughout North Europe and beyond as they travelled and intermarried. DD is a condition that produces bumps or nodules (lumps) on the palm and some fingers. As the disease progresses the lumps join together and form small indentations or "pits". This disease makes it difficult or impossible to straighten your fingers to full extent, it also makes it difficult to do such things as clapping, putting on gloves or putting your hands in your pockets, although this disease causes no pain. This could be part of the decline because they would not be able to farm or grasp a weapon; they then would be useless in battles or raids.
My third and last factor is about in-fighting. The explanation for in-fighting within the Viking society is not because of the lack of respect for the law but because they felt as they had to...