Why was Seth Rich killed?

On July 10, 2016, at 4:20 AM, DNC staffer Seth Rich was found suffering from gunshot wounds at an intersection near his home in Bloomingdale, Washington D.C. He died at a local hospital a few hours later. No arrests have been made so far in the subsequent investigation. While the general assumption is that Rich died in a failed robbery attempt, some evidence, such as the fact that his belongings were not taken, gave rise to the theory that his murder had to do with his position at the DNC. When Julian Assange mentioned Rich a month after the murder these suspicions amplified, since Assange's words were interpreted by many to mean that Rich was a WikiLeaks source.