I agree with others who have said its about a killer. I think that he's a sociopath, so in general, he's incapable of empathizing with other people's emotions. But when he is kidnapping and killing his victims, they have emotions that he can relate to: fear, anger, disgust... so in the moment of killing them, he actually feels closer to humanity. He has a "hybrid moment" - he feels part monster, but also part human. And he is so moved by this connection he starts to get choked up.
I agree with others who have said its about a killer. I think that he's a sociopath, so in general, he's incapable of empathizing with other people's emotions. But when he is kidnapping and killing his victims, they have emotions that he can relate to: fear, anger, disgust... so in the moment of killing them, he actually feels closer to humanity. He has a "hybrid moment" - he feels part monster, but also part human. And he is so moved by this connection he starts to get choked up.