Overpopulation argument fails — if it justifies killing the unborn, it justifies killing anyone

Mike responds that the overpopulation argument, if valid, would also justify killing already-born people. If overpopulation is a reason to kill unborn humans, why not apply the same logic to existing adults? The argument proves too much, exposing its incoherence. The question always returns to SLED — what morally relevant value difference justifies killing an embryo but not an adult?