I'm not Sarah

"I'm not Sarah!" she screamed at the room behind her. She couldn't get her coat and galoshes quickly enough. She'd talk to the agency, she'd make them understand. This guy was nuts.
"Sarah, I need some water," he moaned. She rolled her eyes and turned back towards the door.
"If I bring you water, will you stop calling me Sarah?" she heard him whimper. She hated days like this. He was so pathetic, it was easy to believe that that other times he wasn't truly evil and crazy.
She went to the kitchen, barely able to hear whatever he was saying now. She filled up his glass, only half full and headed towards his room. His moaning had stopped and she peaked in. He was in his bed, almost asleep. She debated if going in was a good idea, why not let him sleep?
"Sa...whoever you want to be called, do you have my water?" he asked. She sighed and pushed open the door.
"Yes, and my name is Megan," she handed him the water but continued to hold and guide the glass to his mouth. She almost took pity on him, except she knew this wouldn't last.
"You hired the agency I work for to have me come and help you. Any of this ringing a bell? I've been here roughly two months now." She watched him for signs of his anger rising.
"Well where's Sarah? Shes nicer to me than you are" he grumbled. She about dropped the glass.
"I dont know" she replied. It was, at least, part of the truth. She knew telling him that his daughter Sarah had run away would only send him into angry hysterics, but it was hard to always be called Sarah. At least he wasn't arguing with her that she was Sarah this time.
"Well find her damnit," he moved his arm like he wanted to hit the bed he was laying on, but it barely moved. She glowered at him. Here it comes.... She braced herself for the verbal chaos and turned her back on him.
Nothing. What the...?
She turned back. He was asleep. She gently took the glass from his hand and walked to the door. He began to snore. Almost there. She closed the door as silently as possible and leaned her back against it, closing her eyes. Flashes of Misery by Stephen King ran through her head. She shook her head to remove that thoughts. Not a good idea, however tempting.