The reason I give Darksiders a boost to Resillience and not a setback it's because I imagine the Light Side to give more discipline over the mind, while the Dark Side to have more discipline over the body (which goes in line with that FFG did with Strain/Wounds for LS and DS users).
It don't think the Dark Side makes you sick, I think it makes you look sick while at the same time giving you power and resistance over the pain you use to fuel it (like Kylo Ren hitting his wound).
Cool, just wasn't sure. Wafrog mentioned setback to resilience and then you gave it a boost. Thought maybe there was a miscommunication there.
I honestly could see it done either way as far as the evil doers being sickly trope goes.