Because only God, as the omniscient omnipotent author and giver of life, and who alone knows what every single effect will be of a life and even of your next thought, not only for this life and for eternity, has the right to take innocent life for any reason. For He alone can and will make everything work out for what is Good and the good of those who choose Him over sin. (Rm. 8:32)
Also, the instances of God commanding children, infants, and pregnant women to be killed were those of, after much longsuffering by God - who is grieved by man (Genesis 6:5,6) to whom He gave good things and good laws (Psalm 19; James 1:17; Acts 14:15–17) while not needing anything from man (Acts 17:25) - were those of executing generationally wicked nations.
Thus placing a stop from their infectious cultural replication, and delivering the innocent from becoming like their parents, while even the guilty will have lesser punishment than if they had been allowed to continue in rejection and defilement of conscience. (Luke 12:47,48; 20:47)
Likewise is God executing wicked souls directly, as is His allowing such to do evil, which can be a judgment on others for disobedience, (1 Co. 11:32) or for chastisement for character sake, (Job) as well as in patience that they may repent.
In short, the presumption that a omniscient and omnipotent, just and merciful Being was unjust in allowing evil or in sending judgments presumes omniscience and omnipotence on the part of the one who is judging God (not that I am not guilty of that, at least as a emotional yet irrational reaction).
For laws which deem something as evil are based upon principals related to their nature and what their effects will be, and with intent and accountability factoring in assigning degrees of guilt. And thus acts such as stealing are crimes of society, but in just wars they are justified in the light of intent and the effect of fighting an evil aggressor. Likewise allowing one to choose wrong answers or actions, and experiencing effects are justified under certain conditions.
But when a Being knows what all the effects will be of even our next hesitation (and all that we do or do not do results in effects in varied way and to varying degrees) not only in this mere speck of our earthly existence as mere specks in humanity and the universe, but for eternity, and can and make all work out for what is Good, and just, yet showing mercy and grace, then mere exceedingly finite ignorant man can hardly sit in judgment on such a Being.
Thus, man cannot rationally impugn iniquity to God for His actions or inactions unless one presumes omniscience and omnipotence for man. Which makes man to be as God, which is simply irrational.
And the reason man is wrong for taking innocent life as well as to judge God for doing so is because man neither,
*created life nor can give it in the eternal realm;
*knows all about the subjects at issue, including all their thoughts, intents and motives;
*can foresee, in all depth and detail, how a life will turn out and effect, now and for the future;
*has the power to orchestra all the actions and works that man is allowed to engage in to at the least ultimately work out for what is Good.
And thus man cannot presume to know that,
*It was not better in the light of all that can be know that, as regards the Flood for instance, God acted as He did, including delivering the innocent from becoming like their damnable parents, and rehabilitating the earth. And making animals to be fertilizer and fuel for man.
*The creator who gave life and all good things has not the right to take it, and cannot recreate it.
Conversely, you cannot rationally impugn iniquity to God for taking innocent life or man and animals unless you as establish that the God you accuse of injustice does not possess all these attributes, but that you do. In which case you are not dealing with the God of the Bible, not the reality of the human condition.
See Theodicy in dealing with alternative proposals as to how God should have operated.
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