So someone has been struck down by malicuim in a town that doesn't yet (as far as anyone knows) have demons in it? Someone didn't think their plot through too well. On another note, Wyne is always so adorable.
Well, MC did just poured out a bottle of it, so there technically is a source. Its just that its not at all a good source. He literally just demonstrated its effects against his creations, and just got done demonstrating being in proximity to not just a small amount, but a HIGHLY CONCENTRATED amount of malicuim, and is still completely fine.
Plus, with malicuim being partial based and not like, energy base or something, it should first effect those closest to the source, then spread out from there. That guy was in the middle of the crowd. And literally no one else is having an issue (so far), especially no one nearest the stage where the malicuim quantity should be the highest.
That all being said though. People tend to become bigger idiots in relation to the size the gathering they are in. A crowd this size likely won't think or act rationally to the claim of him having malicuim poisoning, and will begin to panic or start to doubt MC's product's true effectiveness.
But this has also possibly created the literal best scenario to market his main goal, and through direct example too. The people do not know that they are currently, and have been in proximity to literally some of the strongest, well known demons and have had no issues while mingling in their ignorance. If MC can take control over the situation right away, ensuring that this cannot develop into a panic, then he will pretty much win right here and now. Because often times, one of the best way to demonstrate that something is actually safe, is to have people experience it and decide for themselves to trust that it is safe, before they become aware of it's potential danger. There may be some initial backlash to being deceived, but it will be hard to deny what they experienced already.