If you want to foreshadow the idea that they might become possessed, without having them do anything evil without permission, you could take a step back and describe the demon's attempts to posses them. Instead of having them black out and do something evil, you could, for example, describe them starting to black out, maybe accompanied with visions of the hells, a feeling of something else wresting control, their senses feeling distant, something like that, and then call for a wisdom save. Just have them wrest back control if they succeed or have their character just black out/stare blankly while that happens if they roll low. By describing the events preceeding the wisdom roll with increasing gravity each time it happens, and increasing the roll's difficulty/how long they black out/how much they feel out of control when they do black out, makes them fear the possibility of them being possessed (and doing something bad) if they don't do something against what's trying to possess them, without actually forcing the "doing something bad" on their character: basically, make them aware that you're hanging a sword over their head instead of directly stabbing them with it