He is clearly a psychopath. So to you and me, it's impossible to understand but my best guess is that he would feel something similar to what we might feel "killing" a character in a video game: no compassion, just a curious interest.
Aside from the vehement distaste I feel towards these abominations, they're also fascinating, in that they seem alien to me.
How does one come about? Does it feel and think as I do? How different is its experience of the world as compared to mine? Is it different on the same level as the hypothetical difference in perception between the sexes? Or is it further removed—as the perception of the world would be different for a bat, say.
Does it have a purpose in nature to perform some kind of function? Or is it a failed experimentation (mutation) on nature's part; over-optimizing certain things leading to "unintentional" suboptimization of other things? Or has it more to do with nurture rather than nature?
The fascination is directly proportional to how hard it is to comprehend this.
Trying to understand why a sane person would commit such crime is impossible, so I personally consider that what we have heard and seen is not the behavior of a sane person. Trying to frame it like a video game is trying to impose rules of sanity to explain insanity.