That "new safety feature" was a foolish attempt to patch the terrible design of the reactor. (see my other post)
Everything about that situation was terrible: the badly design reactor, the laughably-bad "safety feature" try wanted to use as a workaround, the untrained operators that were ordered to run the test, and the Soviet politics that caused the foolish rush to complete the test. All of these factors "caused" the disaster. The combination of these failures created a situation that was far more dangerous than a single design problem or human error.
Everything about that situation was terrible: the badly design reactor, the laughably-bad "safety feature" try wanted to use as a workaround, the untrained operators that were ordered to run the test, and the Soviet politics that caused the foolish rush to complete the test. All of these factors "caused" the disaster. The combination of these failures created a situation that was far more dangerous than a single design problem or human error.