If this repeated beating continued, her life would be in danger. Even so, the reason that she was still alive was not because Stafan was controlling his strength. It was because the impact was absorbed by the mattress. If she had been beaten on the hard floor, she would have already been dead.
Stafan didn’t hold back his strength not because he knew this, but because there would not be any problems even if the woman were to die. If he just paid the cost to have her disposed, then everything would be taken care of.