Santana slaps Finn, because he outed her in the school's hallway. Some people overheard their conversation and it was used in a television spot against Sue's campaign.
My personal opinion:
All Santana did was say nasty comments about the way Finn looks. No one's gonna think he's actually an orca though.. Outing someone is never okay. It changes her whole life. It should be her choice when to come out. Nobody deserves this. Besides, Karofsky was worse to Kurt. He physically pushed him into the lockers and stuff, but Kurt never outed him. What Finn did was really low. He totally deserved that slap.