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.