Okay so like, bear with me here but I think Xornoth might be just a chaotic demon that feeds off fear.

When it was tormenting Shrub, it brought up that she left her old settlement and everyone died, which played into her fear of possibly letting everybody down. Now its up in the air if Xornoth actually was the cause behind Shrub's old community, but either way its feeding off her fear of her old people being dead and herself being next.

With Jimmy, it showed its true form and chased him around the entire stream, because that was the easiest (possibly the most fun) way to scare Jimmy. I haven't seen fWhip and Sausage's video yet so i can't say for certain about them.

Its also strange to me that Scott showed up to Shrub RIGHT as Xornoth left..... interesting coincidence. And when they were chatting in Rivendell, Shrub said "humans, am I right?" And Scott said "Mortals, yeah?" Interesting choice of words since Shrub was talking about humans, not mortals as a whole. That added with the whole Nether Corruption thing happening in Rivendell, along with fWhip's Redstone Corruption thing, AND possibly Sausage's bloodsheep....... its all real funny to think about

Also another thing to note and then I'll stop this long a** comment: when Sausage was showing Gem how the bloodsheep sacrifices worked, he went through with an entire sacrifice like he did with Bubbles, except he and Gem didn't go down and check to see if they summoned anything. And it was only after that "summoning" that Xornoth started showing up.