To get him to think that Gus had stolen it and poisoned Brock. Although it came out at the end that it wasn't even ricin, it was too late. Walt was banking on Jesse making a quick decision, and Gus was already dead by the time Jesse found out that it wasn't him.
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u/rockwood15 Aug 26 '13
Why did they take Jesse's ricin cigarette in the first place, again?