I shall comment on the treatment of thralls in Iceland.

The Legal Status of Thralls

The Icelandic Grágás (Grey Goose Laws) comments on the topic of fettering criminals as making the criminal sub-human. It was illegal in Icelandic culture to chain an innocent man. Given that thralls were literally or figuratively in chains at all times, the idea here is that they were by definition sub-human.

According to medieval law, thralls had a low status, and if a thrall acted in an unacceptable way, from the owner's point of view, he had reached the margins of what was humanly conceivable, as in the case if a thrall killed his owner. If the thrall tried to escape afterwards, but was caught, his pursuers had to cut off his arms and legs, and let him live as long as he could with no other possibilities to move except by crawling along the earth. [1]

The idea here is that in Norse culture that being in chains (whether literally or figuratively) made you no better than an animal, and the prescription of converting a fugitive slave into an animalistic state of being reinforces the idea that thralls were considered privileged sub-humans.

Nevertheless, there were some limits on how thralls could be treated due to their status as items of property. For example, should another man rape one man's thrall (male or female), he would have to pay compensation.[2] However, the fact that a thrall could be killed with impunity provided that the killing was announced the same day [3], and given the illegality of homosexual sex between freemen [4], this fact only seems to reinforce the legal status of thralls as sub-human objects or animals.

The Real Treatment of Thralls

In Landnamabok (The Book of Settlements) that tells the tale of the settlement of Iceland - Aud the Deep Minded frees her slaves and they become great men with large tracts of land.[5] This may indicate that slaves could be treated quite well by their masters but a few things must be bore in mind. Firstly, Sturlubok, the manuscript referenced above was written by Sturla Thordarson - a descendent of Aud - who would likely want to present her as a good Christian - freeing her slaves. Secondly, Icelanders were desperately trying to prove during the Middle Ages that they weren't descended from slaves, and would likely have revised their origin story to reflect this desire.

DNA evidence from modern Icelanders indicates that the female line has Celtic roots, whilst the male line has Norwegian roots. This appears to indicate that during the settlement period (870-930 CE) that men were travelling via Ireland and Scotland, picking up women (presumably without consent), taking them to Iceland, and then impregnating them (presumably without consent).

Sources

[1] D. Pelteret, 'The Image of the Slave in Some Anglo-Saxon and Norse Sources' available here: http://www.tandfonline.com/doi/pdf/10.1080/714005239

[2] K. Gade, 'Homosexuality and Rape of Males in Old Norse Law and Literature', Scandinavian Studies 2 (1986), pp. 124 - 141, p. 136.

[3] Ibid.

[4] Ibid.

[5] S. Thordarson, Landnamabok, available online at :http://www.snerpa.is/net/snorri/landnama.htm

EDIT: Formatting.

EDIT 2: In my mention of killing another person's thrall above notice that I have not used the word 'murder'. There is a difference between a 'killing' and a 'murder' for slaves and freemen alike in medieval Icelandic law.