Mask of Brutality's cost is optional. This means that if you don't wish to keep MoB on the field, you simply opt not to pay the 1000 during your standby phase and its destroyed.
Most cards that have a cost are considered optional in this sense. An example of a cost that IS NOT optional is that of the Archfiends, once you summon it, you MUST pay the cost as long as you have the lifepoints available.