Completely false argument. If an enemy moves civilians into an authentic military target, it is licit to attack it because it is still a valid target. Targeting something which is not a valid military target because it kills enemy civilians is completely illicit. So is killing an innocent person because he may be an enemy in the future.
Guatemala judge orders former President Otto Perez Molina detained for hearing in fraud scandal (AP)
from the article: Police release rabbi arrested for inciting to kill non-Jews