The Grail is a worldwide organization dedicated to one purpose: safeguarding the lineage of Jesus Christ until Armageddon. It has offices around the world, and men of power in its employ.
Poussin was a Frenchman, and not particularly liked by his underlings. He traveled to San Francisco on the orders of the Grail AAllfather D'Aronique to track down Jesse Custer. He was assisted by local agents Featherstone and Hoover, who took an instant dislike to him.
There was another reason for Poussin to be in San Francisco with Hoover and Featherstone - he suspected a conspiracy within The Grail. And when the Grail's Sacred Executioner, Starr arrived, he found out that he had been right.
And then Starr shot him dead.