ZH_zh_obs-tn/content/17/09.md

David ruled with justice and faithfulness for many years

This could be translated as, "When David ruled the people, for many years he did what was right and fair, and he was faithful to God."

toward the end of his life

This may be translated as, "when David was older" or "later on in David's life."

sinned terribly

That is, "sinned in a very evil way." David's sin was especially evil.