It's just fucked up that the prick would even want to continue after the first few dogs.
I think that after the first, he got into a stage of no return, kind of frenzy, no matter how disgusted he may have been with himself. Could probably lead to trauma afterwards, but how did he get himself to kill the first one? I've wondered about that when I read about Nazi killings too, how did they get started. The first is the threshold I guess.
Culling of animals is disgusting IMO. Lots of weird psychology going on in how people can just go on with things like that, be it animals of humans.