Did God's moral principles change with the change of OT times to NT times?

Did God's moral principles change with the change of OT times to NT times ?

Doubtful (James 1:17), but our perception of him did, though, through the coming of Christ, God's final revelation to (and in) man (John 14:9; Colossians 1:15; Galatians 1:8). What once was tolerated (Matthew 19:8) will not be endured any further (Acts 17:30). In the latter verse, the apostle speaks of idolatry (as can be easily deduced from context); even though the pious Israelites did not physically worship any such images, their incorrect perception of God, reverberating throughout many passages of the Old Covenant, and influenced by remnants of their own ancestors' pre-Abrahamic paganism (Joshua 24:2), constituted precisely its spiritual equivalent, prohibiting them from either knowing or worshipping the true God (John 4:23-24, 16:3—note that the preceding verse, 16:2, explicitly mentions religious executions). When asked such questions, concerning the various divergences between the two Covenants, Basil of Caesarea, echoing Paul's reasoning from his first letter to the Corinthians (13:10-11), used to reply that, while sucking at breasts may be permitted unto babes, the same courtesy is not, however, extended to grown-ups, and understandably so.

While I understand why we would question God for killing seemingly innocent people in those 2 stories, I think we need to remember that the primary focus of those 2 stories is to demonstrate God's protection of His prophets: Abraham and Elijah. In the narration of the stories then, we shouldn't expect God to explain the morality of His actions, keeping in mind the following factors:

1. How this is similar to how God told Joshua to kill everyone (including women and children) when taking Canaanite cities in the promised land or in the (possible) innocence among some of the firstborns in Egypt killed in the 10th plague. Most commentators are very uncomfortable with God's action in Canaan, in what seem to look like genocide, but try to mitigate this in terms of Gen 15:16: God's giving them time.

2. That out of His foreknowledge and omniscience God could have taken into account His personal dealings with each of the 50 men in the Elijah story. This is maybe similar to how Abraham asked God whether He would destroy Sodom if there are more than 50, 40, 30, 20, 10 righteous people (Gen 18:16-33).

3. That in the Abimelech story, it was a threat not carried out, and that Abimelech was dealing with God as either a King or a head of the household where in ancient practice at the time puts the King / head of the household as a rightful representative of the fate of people "belonging" to him. Thus, for the original audience, they wouldn't see God as unjust.

4. That in the Elijah story, we may see those 50 men as war combatants in God's war against a rebellious king of Israel, just as God became enemy to Samaria when He executed judgment in the Assyrian exile.

5. That in these 2 stories we are talking about God's actions, not human actions against another, which are regulated by Deut 24:16. Morality rules for human are only a subset of God's justice; thus, we cannot limit evaluation of God's justice from Deut 24:16 alone. See this article for more discussion.

6. That life on earth is temporary. If we are killed as part of God's greater plan so good can come from evil, God will reward us in the afterlife.

Of course the above factors are not completely satisfactory to answer your good question, but in my mind still gives room to allow consistency with the moral concept you cited in your first paragraph, especially given the narrative perspective of the stories where we should not infer too much from them. I think when it comes to knowing God's moral principles, His direct commandments in the Pentateuch and the Psalmist's teaching about God are a much better source to argue for consistency between OT and NT. Furthermore, as I argued in point #5 above, the two stories are not proper illustrations of Deut 24:16 or of Ez 18:20, which is the basis of the moral principle you cited in your question.

CONCLUSION: Given the larger factors that we have to consider about the morality of God's own acts as opposed to God's moral principles imposed on us humans, the 2 stories you cited are not a valid basis to support the contention that God's moral principles have changed from OT to NT. Deut 24:16 is still applicable for us in both testaments, but is not applicable to God.