How could Robert Rodriguez ruin a film that Tarantino nailed in the first half?

Rating: Null/10 (3 January 2019)

From Dusk Till Dawn annoyed me in a variety of ways, particularly because the first half was so great and the latter totally destroyed the film. The first half felt like one of Tarantino's original stories, mixing many of the interactions found in Reservoir Dogs and Pulp Fiction. I honestly loved it. Quentin Tarantino and George Clooney did a stellar job with their performances all throughout, and I thought they melded together extremely well. Along with that, the music, screenplay, and dialogue all meshed together very wonderfully in the first half.
But, unfortunately, in one of the most disappointing decisions in the history of cinema, Robert Rodriguez decides to take part in the script once the plot takes place at the bar in Mexico. You know, the Titty Twisters? It was funny, to a certain extent. But then, vampires (who definitely look and act a lot like zombies) take over, and everything becomes goofy, ridiculous, and way over-the-top. Even if that was the point, it did not mesh well at all with the first half of the film at all, which many could argue would have made a great feature-length Tarantino feature. The vampires were so dumb and goofy, and although I laughed, I was cringing more than anything. I cringed at the fact that a perfect film was ultimately ruined. Why?