Moments when Morty seemed truly dangerous to Rick were few and far between when the boy had been human, and Rick was grateful for that, but now.. Now Morty was all barely leashed violence and power, and Rick doubted that, if Morty had truly wanted it, he could've stopped the teen from killing him.

Or worse, sinking those fangs into his flesh and taking what they both wanted.

Canon divergence from Big Trouble in Little Sanchez wherein Rick doesn’t transfer his consciousness into a younger clone of himself to slay the vampire and Morty pays the price.