They are trying to get rid of Trump, clearly. If Russia hacked the DNC to support Trump, it's easier to believe that Trump is colluding with the Russians and that's why they are supporting this. There is obviously no evidence so they are hoping they can convince the general population with a whole bunch of brainwashing. If you try to get to the bottom of anything mainstream, you just hear about anonymous sources that can never be traced to anything.

Seth Rich did indeed exist -fact
Seth Rich did indeed work for the DNC - fact
Seth Rich was an advocate for Bernie Sanders - fact
Seth Rich did have e-mails tying him to the DNC leak - fact
John Podesta did state that they should make an strong example of the leaker - fact
Wikileaks publicly mourned for Seth Rich's murder last year - fact
Seth Rich was shot twice very early in the mourning last year while the attacker left his wallet, phone, and jewelry on his body. This murder was called robbery and was not highly covered in the media - fact

Seth Rich was more than likely the DNC staffer who leaked information to Wikileaks in protest of HRC/DNC's rigging of the election against Bernie Sanders and was assassinated to be made an example of.

I can provide a source for everyone of those but you can easily do a Google search and find multiple articles covering each point. If not, let me know and I will provide them.