I believe I have broken the quest for rakghoul serum. I threatened Zelka I would tell the Sith about the republic fugitives he was hiding and had him pay me. Now he won't talk to me about the rakghoul serum and neither will Dax the Hutt, or Zelka's assistant. So I was curious as to if there was any way to reset Zelka on the pc version or if the only option was to start over?

I think as long as you didn't sell the serum to Gurney, you can still give it to Zelka once you find it.

I tried and nothing comes up in conversation with him. So I think I need to restart the game again unfortunately

Is this for PC or Xbox? (sorry I should have clarified earlier) if it's for PC you can probably find an override command/mod for it on here (just use the search function); otherwise try re-opening the locked door in the lab and when Zelka scolds you, tell him that his secret is safe (this should work for both PC & XBOX) that SHOULD reset the in game variable and allow you to give the serum to him.

If that doesn't work then you may have to restart the game since the game figured you were going to take the Darkside route anyway.