Middle Passage
Slaves were often tortured as punishment if they did not follow the human traffickers' orders or if they seemed disobedient in any way.  For example, enslaved people who were too depressed or sick to eat might be beaten or whipped.  The worst forms of torture were for enslaved people who tried to rebel (fight back).  For example, one slave ship captain punished an enslaved person who tried to rebel, by murdering him; then he forced two other slaves to eat his heart and liver.