Why isn’t Bruno afraid of Lieutenant Kotler at first?

Bruno isn't afraid of Kotler initially because his father is Kotler's superior. He is, however, uncomfotable around him.

Lieutenant Kotler was the young officer whom Bruno had seen on his very first day at Out-With, the soldier who had appeared upstairs in their house and looked at him for a moment before nodding his head and continuing on his way. Bruno had seen him on many occasions since - he came in and out of the house as if he owned the place and Father's office was clearly not out of bounds to him at all - but they hadn't spoken very often. Bruno wasn't entirely sure why, but he knew that he didn't like Lieutenant Kotler. There was an atmosphere around him that made Bruno feel very cold and want to put a jumper on.