Del Varner was an illegal tech-runner.
From 2252 to 2257, Varner continued his nefarious career of tech-running which took him to many places including in Vega System and Proxima System. He also did a lot of business with the Centauri Republic. He did get caught several times and by 2257, Varner tended to avoid authorities but was also heavily in debt.
In 2257, a lucrative opportunity to settle his debts and get paid was presented to him when he came into possession of a changeling net. Willing to risk detection by local authorities, Varner smuggled the device onto Babylon 5 to sell to a Minbari assassin. Varner was supposed to meet a transport before it arrived at Babylon 5, but they were unable to make the rendezvous and Varner brought it to the station.
When the Minbari assassin arrived at Babylon 5, he killed Varner and assumed his identity. A Babylon 5 technician, Eric, recognized the assassin-as-Varner and was killed to protect the secret. The assassin continued to operate on the station for several days in the guise of Del Varner, carrying out his mission to upset Babylon 5's purpose in creating peace and cooperation between his people and the alien governments.
Station Security - led by Michael Garibaldi - found Varner's body floating in a water tank in his quarters. He had been in the tank for some time, judging from the level of decomposition. Dr. Benjamin Kyle's autopsy reported that Varner had been dead for several days, even though witnesses said they saw him walking around recently. It was while reporting the results to Lyta Alexander - who was actually the assassin in disguise - that the assassin tried a second time to kill the ambassador. However, the real Alexander witnessed the event, and the truth of the set-up was made public.