That confused me too. After watching the series a second time I am of the opinion that Walt wanted Jessie's help killing Gus, But Gus had been grooming Jessie who was starting to like Gus a lot. Thus I think Walt poisoned the boy and convinced him Gus had done it. Thus he got Jessie to hate Gus and help him murder him. By the way, it was not Ricin but berries from the Lily of the Valley plant he had in a pot out on his patio. The producers error-ed though as the plant had flowers on it. . not berries!
Thanks. I re-watched it too, and came to the same conclusion. it was clear that Walt was worried that Jesse was too drawn to Gus. I had missed the point about the other plant -- realize now that it was sitting next to Walt's pool, and he had used to mimic ricin more safely. All a tad too subtle
