He bought Twitter because he (along with the Saudi royal family, the monarchy ruling Qatar, and the larger worldwide cabal of authoritarians) thought he could use it to control public discourse in the same way oligarchs bought up newspapers, radio stations, and TV stations in the past. He's killing it because he never understood what most people liked about Twitter, he only understood what he liked about Twitter (getting a lot of attention and being able to be shitty to people without having to actually socialize).