Personal I think what he did wrong was changing the entire company system and rules .in fact he just changed everything so many restrictions and I guess that's why majority of advertising firms stopped advertising there. He just fucked up thinking everything could go his way. He should have followed the system a little and make changes more slowly. Even tho he is a genius ill say its not a very smart move from him 'Mr Twitter destroyer' 
well at the beginning of the acquisition he keeps saying that twitter is fully infested with bots, and thats true honestly. I guess thats why he changed things around a lot nowadays with the twitter even go such lengths of renaming the entire company and logo.
but the question is, is that really a bad decision coming from elon though? by changing around the system he might have reduced the cost to make the company runs.
also he is trying hard to eliminate the bots to go as far as publishing the algorithm to find out bots in open source with hopes that it could be further advanced by the open source community.
many said that elon is doing 4d chess right now.