Why did God kill people?
Why did God allow for the striking-down of people in the Old Testament? How is this reconciled with the dogma of a loving God?
Protestants and modern-day “Jews” don’t have an answer for this — one beyond dualism or “mystery”, I mean. But the Church does.
There is mortal sin, and there is venial sin (1 John 5:17). Mortal sin — willful and of grave nature — separates one from God, practically killing the soul. Venial sin — all other — must simply be cleansed, and it does not eternally separate us from Him. And this distinction is shown in the Old Testament.
Among mortal sins punished: irreverence (2 Samuel 6:1-7), despair/disbelief (Numbers 11:1-3), and false claims of authority (Numbers 16). These crimes have always been condemned.
Why did punishment change from body-centric to soul-centric? First, it didn’t, because unrighteous people were also kept from entering the Limbo of the Patriarchs, which, after Christ’s Sacrifice, later led to Heaven. Second, temporal punishment was the only way to get at the Jews’ consciences: As liberal scholars love to point out, the majority of Jews did not believe in an afterlife!
God does not desire death, though it can be used to give us the best chance at salvation (2 Peter 3). (I think here of St. Rita and her sons.) Even in the old days, He merely wanted a contrite heart (Psalm 51:17), and He wanted devotion.
There is no change in principle: God is immutable.