What's the point in inseminating something you're going to eat? Was he really THAT uncontrollably horny? I never understood why doujins keep doing that.
Why do people rape other people they intend to kill or vise versa? Because it gives them kicks. I don't see how a monster would be that different. For all we know it delights in the pain and humiliation of others (because having sex isn't all about making children).