I think that the hallucinations of Becca in Butcher's mind triggered the worm-like V-things crawing around under Butcher's skin into transforming Butcher into something else which led him to go ham on Ezekiel and tear him to pieces... It's why I'm thinking that Butcher isn't necessarily on death's door... yet... I think he makes it to S5, but w/ this show, who knows...