Why is Bruno surprised to learn Lieutenant Kotler has friends?

Bruno is surprised because he sees Kotler for the man he is.... a cruel, mean person. The fact that he could have friends.... that anyone could actually like him was shocking. I think it's also evident that Bruno doesn't see Kotler as anything different than what he is. Bruno cannot envision him as a boy like himself.

'Yes, I made many swings myself when I was a child. My friends and I had many happy afternoons together playing on them.'
Bruno felt astonished that he could have anything in common with him (and even more surprised to learn that Lieutenant Kotler had ever had friends).