You mean people that point out shady shit a company is doing? Short sellers serve a very important role in the market and help prevent things like Theranos.
True, but it can be a significant problem for legitimate companies. After going public Tesla had the highest short interest of any major stock at 72%. The other companies that were in the top 5 at the time are all bankrupt or close to it now. Short sellers perceived it as an unrealistic joke, and tried to convince everyone else of that, which could easily have become self-fulfilling.
But they didn't, did they. So what's the problem here? There is no problem with a big chunk of people holding a short position in a stock. Tesla has some big problems that traditional business people get concerned about (e.g. making money).
Activist investor is minority investor who puts pressure on management and generally campaigns to change something about a company - for their own goals.
