To add to the other comment, it was also "weird and undemocratic" to grant the title of Chancellor to a dude who literally tried an undemocratic coup already and whose ideology was almost explicitly about dismantling democracy.

We can't afford reverence to unspoken practices or customs or "good-faith" when dealing with fascists. Fascists are not dealing in good faith, they will use the system of laws and customs you created to get into power, cement themselves there by destroying parts of the system which oppose them, and finally get to enriching themselves as much as possible while sowing death and destruction and bigotry everywhere.

There were strategic mistakes made by the communists which indirectly helped hitler to power (the same kind was ironically made by dems in 2010s). But it was Hindenburg who turned hitler from a wannabe dictator into an actual dictator in charge of a country.