Dr. Dao has a good case against the security service, but less so against the airline

United Airlines
1. United is allowed to remove passengers at their discretion, it is called involuntary de-boarding and it can be done for a variety of reasons, if certain steps such as offering compensation.  Passengers agree to this when they purchase a ticket

2. United requested the passenger leave the aircraft (lawful order from flight crew), passenger refused and security service (Not United employee's or contractors) was called

Legally United followed the appropriate steps, morality is another question

Against Airport Security Service
1. It appears that the security service used excessive force, but I don't think it will help Dao's case that he refused to comply with the officers orders and he tried to run back onto the airplane. I doubt any court would accept criminal charges for Dao's behavior, but he did break the law.

My guess is that this will be settled out of court.