Can a player who is supposed to be a hero character become a traitor by their own will? For example: if the haunt doesn't specify any hidden traitors or others but the hero character wants to help the monster any way is that allowed?
No, a hero cannot become a traitor unless the rules of the scenario cause him to become one.
Allowing that would be no different than allowing a traitor to simply declare "I am a hero". Now, technically a hero can help the traitor if he wants to in a number of ways; but if the traitor wins that player would still lose.