Must mean "required". Use to force the occurrence of actions according to the rules are necessary, unavoidable. If it is inevitable to occur, they must be punished or have bad results.
must mean "need" to use. Forced to take action according to the rules. An unavoidable necessity. If the inevitable happens, he will be punished. Or subsequent ill effects
Must means "to" used to force the action according to the rules. It is necessary to avoid. If there is to avoid the happen, it will have to be punished or bad consequences.