Couldn't find the right way to phrase this properly, but he's pretty much saying that the reason its okay for him to kill yukkuri's but not for Reimu to do it is because he enjoys it so much. "Watching Reimu begin to quiver as the sound of the chainsaw started up" "seeing their wretched deaths - can't get enough of it" etc
Please stop adding the justice tag, this is not a justice story.
The story basically goes, Anon picks up Reimu and Marisa shortly after they give birth. Anon tells them they're carrying a "disease" and gives them ONE pill between the two of them (the pill is actually just a vitamin pill and there is no disease, post #5139). Marisa obviously freaks out and starts blaming Reimu thinking that shes about to die from the "disease". Reimu eventually snaps and kills the kos in anger and Anon kills Marisa. When Reimu asks why she can't be released Anon admits that the whole point of this story was because he likes to kill yukkuris.