O-Dog himself can be seen as this. He mercilessly kills a Korean cashier couple in a liquor store. It gets worse when he steals the videotape and uses it as a method of humorous entertainment for him and his homies.
Adding to the horror is that in the ghetto, there really are people like him on the streets.
The scene where Caine and his cousin Harold are carjacked by a rival gang. One of them shoots Harold in the head, splattering his brains all over the windshield. The worst part is that it was solely done For the Evulz.
Also, the scene where Caine is taken to the hospital afterwards, where he is bleeding graphically and in shock.
The scene where the strung-out addict is begging O-Dog for drugs, desperately offering him anything (his lunch, sexual favors), and O-Dog not only callously kills him, he offers his friends the dead man's cheeseburgers. That's pretty heartless, even for a bloodthirsty maniac.
Even A-Wax calls him out on this.
The quietly terrifying detective that interrogates Caine in the middle of the film, played by Bill Duke.
Caine's death at the end of the film. He saves Ronnie's son, but at the graphic cost of his own life. The unrated version of the film is worse, as it contains a bunch of gore as Caine is shot.