James Dylan was an employee of Helping Hands Viaticals. Unfortunately, he had lied to get his job, and when his colleagues overheard Sherlock's statement that he was "a convicted felon" he lost his position almost immediately.
Dylan returned to confront Holmes as the latter was emerging from the police station celebrating catching the real killer in the episode. After listing his grievances (which included the fact that he was being sent back to prison for violating his parole by getting the job he had had) he shot at the consulting detective. Detective Bell saw the gun and dived in the way as fellow officers took down the assailant. Bell himself was very severely injured, and it seemed likely he might not be able to return to being a detective even when his physical recovery was complete.
This led to the investigation into Holmes and Watson's conduct that was the basis of Tremors.