This question already has an answer here:
Why was Jesse angry with Walter in the last few episodes of Breaking Bad when he was getting picked to be relocated in Alaska?
- He already knows that Brock was poisoned with lily (from doc report), and from that he got to know that Gus didn't poison Brock and neither did Walter (because Walter had destroyed the plant of lily). There is no evidence in the series by which Jesse could be knowing Walter's possession of the lily plant.
- Even if Walter stole ricin, there could be a possibility (which Jesse should have thought) that the stolen ricin can be used for any future endeavors (killing Hank perhaps). I mean what made Jesse so strongly believe that it was Walter who made Huell to pick pocket a ricin cigarette from him (because the second ricin cigarette, made by Walter was found in the Roomba in Jesse's house)?
Also Jesse was pretty sure that Walter only did the conspiracy of poisoning Brock where it was already established and known to him that poisoning was done from lily flower (doc report).
Jesse was definitely strongly believing that it was Walter who ordered for pick pocketing. But why does ricin poisoning matter at all now? Such that he made desperate attempts twice to burn down Walter's house.