I don't see Hitler as insane. That makes it to easy. He sure was not a healthy man, most historians are pretty confident that he suffered from Parkinson's disease shown by tremors like in his last recorded footage during the battle of Berlin 1945. But he most probably was in full command of his mental faculties. As far as the growing of the Nazi party goes. They actually have not been even that popular in public, which is shown by the votes and how many seats they occupied in the Reichstag. They never achieved a majority here and without the help of a few other parties they might have never eventaken over the government. However they did achieve most of it with more or less democratic means.
What really played in the hands of the Nazis was the incopetence and outright naivity of the politicans of that time to react to the fascistic dangers and their militaristic structure. They would bully and threaten any opposition using the SA as goon squad, going as far as to kill individuals. They had a huge advantage in their organisation. The Nazis had a very clear leadership and structure, they simply have been more efficient compared to the SPD and the other parties, as they are naturally not really fond of using terror and fear. The NSDAP however managed it succesfully to build aliances with other fascistic parties like the DNVP and in 1933 the DNVP was simply assimilated by the NSDAP which helped them to reach the majority in the Reichstag.
Votings of the 5. March 1933
12,3 % Kommunistische Partei Deutschlands (KPD)
18,3 % Sozialdemokratische Partei Deutschlands (SPD)
13,9 % Parteien der bürgerlichen Mitte, Zentrum und Bayerische Volkspartei (BVP),
Nationalsozialistische Deutsche Arbeiterpartei (NSDAP), and the Deutschnationale Volkspartei (DNVP) with 43,9 Prozent respectively 8 %.
While the Nazis had quite a lot of the votes, they never had the clear majority and there have been signs that they would start to lose votes with the next voting. With 1933 the economy and political situation in Germany already started to settle down and become more stable which made it difficult for the Nazis to gain more influence. People simply didn't really wanted more goons on the street.
However the Nazis also understood it how to gain support in the monarchic and elite of Germany which had a huge aversion against the Republic. On the other side they had also the fear of a communistic revolution not unlike in Russia. Hitler never made a secret about his intention to fight the communists when ever possible which gave him a lot of sympathies and incredbly huge financial support by some of the richest industries in Germany. In the 1930s Hitler was most probably one of the richest man in the Reich with his own small private army of goons at his disposal. And he didn't even payed taxes. It was really more the support of the elites and industry in Germany that helped him to gain power instead of the population. Albeit he definitely was somewhat popular in the 1930s no doubt about that. But a huge support by the German population like the Nazis claimed? I would say no. If following only he votes, roughly 35%.
Another point was the fact that those individuals with some sort of power, like Franz von Papen and Kurt von Schleicher supported Hitler in to the position of Chancellor in the 1930s. Those individuals havn't been Nazis but their idea was to overcome the Republic. By 1930 already the Article 48 of the Weimar Republic "
(...) allowed the President, under certain circumstances, to take emergency measures without the prior consent of the Reichstag. This power was understood to include the promulgation of emergency decrees (Notverordnungen)", which had wide ranging applications. In 1933 Hindenburg Nominated Hitler to the position of Chancellor which pretty much marked the end of the Weimar Republic, albeit the decline allready happend several years before the Nazis made their way into the Reichstag. One can not ignore the role that the elites and individuals like Papen, Schleicher and Hindenburg played in the takeover by the Nazis. The Weimar Republic actually was already on it's way of becoming a dictatorship with erroding it's democratic institutions and Hindenburg as President of the Reich was already close to a dictator before Hitler came in to power. Hitler, and I think even he said that at some point, just gave the Republic the killing blow.