You asked me the same question, I answered it, and now it seems as if you just disregarded what I said.
You are viewing taking the baby's life as a PUNISHMENT, when it ISN'T. It would actually be a RELIEF from the potential life of suffering it would endure if it was forced to live. Don't you believe in heaven? If a baby was killed before it had a chance to even sin, wouldn't it go to heaven? I don't see the downside here. Instead of making the child suffer, you are giving it a chance to live an eternal life in paradise. According to christian beliefs of course.