His biggest mistake was saying that he would allow Trump back. In the minds of many left-leaning people that makes him a sympathizer in a coup attempt, and those left-leaning people succeeded in creating an extreme amount of pressure against Twitter’s customers (the advertisers) to starve the company.
Musk clearly thinks of himself as creating a platform for free speech, but when it comes to coups there is really no such thing as a middle ground or neutrality.