This analysis, while novel and interesting to me, is not supported by the bulk of events in the early Old Testament. I will name some examples of God personally meting out the death sentence
en masse as punishment for sin in various books of the Old Testament.
1. The Great Flood (Genesis),
2. Sodom (Genesis),
3. The Plagues of Egypt (Exodus),
4. The Exile of the Israelites from the Promised Land (Exodus).
Redemption, mercy, and forgiveness are primary themes in the Gospels, and further continue as major themes in the rest of the New Testament. Aside from the Lake of Fire (Hell), which is worse than being killed for being a first-born Egyptian child, references to punishment are secondary.
As for Paul's opinion of the Old Testament, that is fine, he wrote that opinion. There is no shortage of contradictory and conflicting opinions and facts in the Bible since it is a work compiled, edited, and written by humans.
That said, the mainstream Christian opinion that the Old Testament is vengeful and the New Testament is merciful is prevalent for a reason. Paul cannot retcon, try as he might, the events of the Old Testament.