A young writer is on the run from a street enforcer sworn to kill himself if he fails to kill his victim.

Your villain "distinguishes" the logline, hence, overwhelms the writer. Ergo, like it or not, you've cast him as the main character in the mind of us who have read your logline. That seems to be the consensus impression. It's not what you intend, but there it is.

You are certainly right that villains can be more interesting than the hero. Darth Vader is more interesting than Luke Skywalker. But how does the logline for the first film of the franchise read?

After a spirited farm boy saves a princess from an the evil henchman of a despotic Galactic Empire, he joins forces to destroy the Empire's planet-annihilating Death Star.

The focal point of the logline is Luke, not Darth Vader.

Thanks for the comment Nicholas but why can't the villain be more interesting than the hero? Thrillers depend on unusual villains. The hero always has an arc but why pitch the arc if the villain is what distinguishes the story.

I think the introduction of the idea that the villain will kill himself if he fails in his task makes him the more interesting character, but also makes it unclear at what stage it would be decided that he had failed? Like, will he hunt him for the rest of his life? Or is there some point or place the writer could get to that would mean, inarguably, that he had "won"?