The Benign Violation Theory of humor, which is probably the best one out there, suggests that something is perceived as funny when it is simultaneously perceived as violating how the world “should” work and as benign. Something like the “gun” meme, for example, is funny because it violates our sense of how a joke should progress, and at the same time it’s harmless. 

Racist/sexist/etc shock humor violates our sense of how the world work–in either a “that’s not true!” or “you’re not allowed to say that!” way–and therefore whether you find it funny is based on whether you find it benign, which is to say either you think it’s harmless or you don’t care about the people it harms. (This is the root of the punch up/kick down distinction–jokes that punch up are funnier than jokes that kick down because the people they target are less vulnerable and therefore less likely to experience harm.)