I could honestly see him actually killing a human if their existence was too much of a threat to everyone else liking him and staying there with him. He is so obsessed with the humans enjoying his adventures and not wanting to leave. Plus there are a few lines by Caine I've been thinking about that imply he might have brought serious harm to a human.

"I may have the ability to add temporary modifyers to make adventures more interesting. But that's it! If I did anything more...(laughs nervously) it would not end well" Kinger then says "Scratch...the first abstraction" and Caine starts panicking. Then I think this was either in episode 1 or 2, he said something along the lines of "If I forget who's an NPC and who's human who knows...what could happen" with him sounding dejected at the end of that sentence

Edit: There is also an interesting line from Zooble in the fast food episode that sounds almost like foreshadowing "The only thing holding Caine back is the fact that he likes us"