The demon's ultimate goal seems to be for the hero and the demon lord to fight, it's essentially a demon following the god of protection's plan to a T, meaning he's a demon who fights and dies fighting the hero as well by leading the hero to the "hero's sword", what isn't said is that the hero's sword robs the hero of whatever emotions they have left, and finally makes them a mindless killing machine, if gideon stays in zultan he can do his demon thing with keeping the plan in check, but he knows that if gideon ever comes into contact with his former party it's only a matter of time before gideon figures out the dark side of the "hero" protection, aka that it's slowly eating away at his sister's humanity and emotions, essentially slowly depersoning her, a good portion of this arc is trying to figure out how to seal her divine protection.
As for other events, the demon can only copy someone he's tasted the blood or flesh of, in danan's case he cut off one of danan's arms, but danan is still alive.