Excellent question that identifies one of the most intriguing aspects of the character of Guy Montag. Montag is a very interesting protagonist to study, because he is by no means a perfect hero - he is often bullied and manipulated into doing various actions by Faber and/or Beatty and is clearly not the most intelligent character in comparison with them. What is interesting though is the way that his hands are often described to act of their own accord, as if they were separate from his body. For example, take his first book robbery:

"Montag had done nothing. His hand had done it all, his hand, with a brain of its own, with a conscience and a curiosity in each trembling finger, had turned thief. Now it plunged the book back under his arm, pressed it tight to sweating armpit, rushed out empty, with a magician's flourish! Look here! Innocent! Look!"

Such examples seem to have two purposes. They firstly underline the conditioning of Montag and his fellow citizens. To act against the law was so contrary to Montag's nature that his hand needs to be presented as a completely separate entity from the rest of Montag's body and particularly his brain. Secondly, and linked with my first point, obviously the actions of Montag's hand can be said to represent Montag's deepest repressed desires to rebel against the system that has brainwashed him. At times this causes him to commit morally dubious actions, such as burning Beatty.