Lyle did a mistake when he had caught Emily.

He should realized it when he found Jarod’s letter in his house. “An eye for an eye, a tooth for a tooth,” it was saying. He really should realized it when his house burned.
Now he was standing in Miss Parker’s room, watching almost invisible marks of fight, smelling the chloroform. His sister hasn't appeard in the Center for Five days already and she probably won't come there any more. She wasn't indifferent to him.
He left Miss Parker's house thinking about a revenge.

Maybe it was mistake when he killed Emily.