It’s bad enough that he’ll be sucking the energy from every living thing he come across, but I suspect that it will be his encounter with Supergirl that accelerates his transformation and turns him into that purple creature.
Several individuals have become Parasite in the Superman mythos, but it’s been confirmed that Lost actor William Mapother is playing Rudy Jones on Supergirl, who became Parasite in the Post-Crisis DC Comics continuity.
Hopefully along with these added abilities, this iteration of Parasite will also gain Kara’s weaknesses, meaning that if there’s some spare Kryptonite sitting around, the DEO can use that to neutralize him.
However, while Rudy was a not-too-bright janitor on the printed page, this version holds a doctor title, and judging by the preview, it looks like he’s accidentally infected rather than deliberately exposed himself to…well, whatever is responsible for next week’s insanity.