1. God is always a just Judge. His judgements are absolutely fair and exacting. His rewards and punishments are always fully deserved.
2. We humans may not see it that way, but we have no standing to judge God. If we read the OT and are horrified, the problem lies with us, not God.
3. In the OT, Israel (among other nations) acted as the instrument of God's judgement. Not one sin was committed by the Israelites carrying out God's commands, even when they slew women and children (see 1 and 2 above). Rather they sinned when they were disobedient and allowed the objects of judgement to live.
4. In the age since Christ, the Church acts as the instrument of God's grace. The church is not in the judgement business-yet. Yet the church is not without authority: what the church looses on earth will be loosed in heaven and what the church binds on earth will be bound in heaven.
LOL, easy to say that, but not for the children, infants or animals slain under "divine" authority in 1 Samuel 15:3, for instance. Can animals sin? What did the children do to deserve genocide? By your means of "justification" of such primitive barbarism, even Muslims can claim their Allah to be a just god, whatever that must mean.
It takes extraordinarily stupendous levels of cognitive dissonance to equate that OT god with the protagonist of the NT, when "slay the infants and children, spare no ass! (1 Samuel 15:3)" has to be squared with "turn the other cheek."
And then you have things in the OT mirrored in the Quran such as capturing and keeping women as spoils of war, and so on and so forth, all vile filth that no sane human can justify as moral or necessary.