According to Kershaw the governing parties were headed towards authoritarianism instead of democracy so the Weimar republic really was already failing when #Hitler became chancellor. Couple that with their delusion that Hitler could be controlled and conservatives and president Hindenburg then not opposing the Nazis when they were the only ones that still had the ability to stop him from grabbing power (looking away when the communists and SPD were prevented from participating in the Reichstag) and you had the recipe for the greatest tragedy of the 20th century.