Slater was the First Mate aboard the slave ship
Slater was a racist, had a very obnoxious attitude towards slaves and no qualms about harming them. He killed several of them. Slater was very ignorant, religious-wise, and very likely raped female slaves aboard the ship. This is brought to light in a scene where he offers Davies a "belly-warmer" slave for the night. He views slaves as objects rather than humans; he is once heard yelling at one of the crew members about dropping ship cargo profits by letting a slave jump overboard.
Slater was killed when the slaves decided to start an uprising. The uprising killed many of the crew, Slater was one of the last to die. When Slater saw the Mandinka wrestler approach him, he pulled out a pistol, aiming it at the slave's head. Just before he could pull the trigger, Kunta Kinte threw a dagger into Slater's lower torso, killing him.