Who said, "People I thought were my friends look at me now as if I had horns growing out my head?"
Cates indeed says these lines. His context is the teaching of Darwinian evolution in place of Christian theology. He says this in the presence of Drummond and Rachel. The notion of being able to shatter, to a certain extent, how individuals perceive their sense and state of being in the world is the reason why individuals look at Cates in such a different way. Drummond remarks that the reason why there is so much vitriol in the way the townspeople saw Cates was because, as opposed to murdering a person, his teachings actually went to the heart and murdered the way people thought about things. This is something, Drummond argues, that will bring everyone and their worlds upon him because it strikes at the very foundation of how people are and how they view their own identities. The implication here is that a zealously religious point of reference is highly challenging to reconcile with a scientific one.
The person who says this is the schoolteacher, Bert Cates. When he says this, he is talking to Henry Drummond, who is his lawyer, and Rachel Brown, who is his girlfriend.
It makes sense that Cates would be the one saying this. After all, in the play, he is the one who has been arrested and is being tried for teaching evolution. He is complaining that people think that he is really evil for having done this.
He says that people look at him as if he were worse than a murderer -- more like a devil.