The argument makes no sense because even if it was the case that fatal child abuse is a benefit on society, there are no policy prescriptions that could come from that. If you leave abusers alone and they don't end up killing their kid, well now you have more potential criminals to deal with (because children who are abused don't tend to turn out well), and that also has a cost.

Not to mention the fact that someone who is fked up enough to kill their child when they could just abandon them instead (which is also illegal but not as bad) is probably a threat to society in many other ways.