When the haunt begins, shall the betrayal reveal to everyone their stats - might, speed, etc. or they should keep them secret?
All information in the traitor's tome is for the traitor's eyes only. Generally, this includes monster stats as well. If the heroes are allowed to know it; then it would be written in their scenario rules as well.
However, because die rolls need to be made publicly; the first time a monster actually uses one of its stats; the heroes will find out what value that stat is. Though many scenarios have the monsters change stats based on number of turns or some other value; in which case the heroes will only know the monster's current stat; not what it might become later.
From the official FAQ:
When do the heroes find out the traits of a monster?
When the monster has to roll dice for a trait. All dice rolls are open.
How much do you have to reveal about what you are doing to the other side?
You should announce the purpose of any action you are taking to the other side -- for instance, "I am making an exorcism roll now," or "This roll is to see whether the house blows up." You don't need to reveal what number you need to roll.
Also see this question dealing with when you are allowed to know whether or not a monster actually has a particular stat.