It struck me as genuine, the breath hitching and hyperventilation is quite hard to fake, try it yourself and you'll see.
I think he is upset, he wouldn't have called 999 in the first place if he wasn't affected by it. And I think that's another thing that's hard for us normies to accept, which is that evil people are just as complex as non-evil people. They can also feel sadness and remorse and all the rest of it, see Dennis Nilsen who wept bitterly over some of his victims, there are very few people who are straight up 100% evil all the time. It's just that there are some acts which are so evil that they cancel out everything else about a person, and torturing a little child to death is definitely one of those things.