Poor Stephen Mallory got off to a very rocky start in this book. In fact, when we first meet him he's the would-be assassin of Ellsworth Toohey. Mallory wasn't a political malcontent though. Mallory was a sculptor. Since we don't run across a ton of artist assassins, that one needs some explaining.
Mallory was a gifted and original sculptor, but, like many famous artists before him, Mallory found his work rejected by society. As a result, he became disgruntled and decided to take out Toohey, since he blamed Toohey for influencing society in bad directions.
Mallory has a lot of anger issues when we first meet him, but Roark acts like a one-man rehabilitation program and gets Mallory to cool his jets.
"Because [...]" Mallory roared suddenly, and then his voice broke, and his head dropped, and he finished in a flat whisper: "because I've spent two years" - his hand circled limply indicated the room - "that's how I've spent them - trying to get used to the fact that what you're trying to tell me doesn't exist [....]" (2.11.143)
After finding personal and professional salvation through Roark and Roark's life philosophy, Mallory becomes a loyal Roark devotee and does work on all his future projects.