He wasn't expecting Claire to step between him and Sylar. He wasn't expecting Claire to try to defend that monster. Something was wrong, something was very, very wrong here, and the idea that he was second guessing himself about Sylar was making Peter angrier.
Then he saw Claire fall.
"Claire!" he shouted, reaching for her. The moment of panic jarred him, and his hold on Sylar was broken for an instant as Peter tried to catch his niece.