Hmm, that is a complicated question XD He is the person who suggested the mission to destroy the facility in the first place and I'm fairly sure that he knew there would be many children there that he would have to kill. He didn't particularly want to kill them, it was more out of necessity. He had to kill them because otherwise they would have grown up to come after him. So he did it more out of survival instincts and because he didn't want them to grow up like him.

\o/