I have heard that slaves would box each other but I was under the impression that this was an activity confined to the slave quarters. Did southerners actually force their slaves to fight to the death? Was this widespread or considered sport? Who would spectate these bouts?
I know we should usually link to academic sources, but this question was actually directly answered by one of the top living historians of slavery today, David Blight. I think a quote from someone as eminent as he is in our field should meet /r/askhistorian's standards, even if it comes from Slate :)
"As David Blight, the director of Yale’s center for the study of slavery, told me: One reason slave owners wouldn’t have pitted their slaves against each other in such a way is strictly economic. Slavery was built upon money, and the fortune to be made for owners was in buying, selling, and working them, not in sending them out to fight at the risk of death."
Now, slaves did engage in wrestling and boxing and other physical competitions, as the article notes, but fights to the death were very unlikely to have been permitted.
Citation: David Blight quoted by Aisha Harris in her article "Was There Really “Mandingo Fighting,” Like in Django Unchained?" at Slate.com, last accessed at http://www.slate.com/blogs/browbeat/2012/12/24/django_unchained_mandingo_fighting_were_any_slaves_really_forced_to_fight.html on 10/7/2015.