Part of the Golden Rule (Lev. 19:18) is concern for other people. One way to view the somber warnings in the common Torah reading בְּחֻקֹּתַי Bechukotai (“in My statutes”) is God is concerned about the world so much that its lifeline — Israel — has to remain pure.
The usual complementary reading to Leviticus 26-27 from the Prophets is Jer. 16:19-17:14.
It is only God Who can restore life from death, not only the death of a person but the death of a nation. God warns Israel, and He will destroy their nation if they walk in idolatry. But He will also restore their nation if they will humble themselves and accept their guilt.
The topic of Leviticus 26-27 is God’s anger: the just anger that comes when His people do not follow the way He has laid out for them. God reveals the blessings He will give them if they obey Him and give their hearts to Him and the curses that will come upon them they disobey Him by running towards other gods.
The book of Leviticus is not written in chronological order but in thematic order. God may also repeat a point several times as an emphasis on that particular point.