At first I was thinking that Gray Matter was the spark he needed to go back and try to fix things... However, I'm now thinking that he's blaming Gray Matter and Gretchen for everything. Something happened on that family vacation with Gretchen which caused him to leave her (and his research that Gray Matter is based on) behind. If you think about it, Jack stole his "life's work" i.e. money/blue meth, as did Gretchen and Elliot with Gray Matter. Why did he leave Gretchen in the first place and why did he sell his stake so cheaply?
To have them say that he only contributed the name likely irked that ego of his back to the root cause of his need to 'break bad' in the first place. I literally have no idea how things will end but I'm pretty sure there will be a Jesse/Todd finale, as well as a Walter vs. Gray Matter incident.
I don't think Jesse will kill Walter, but he may leave him somewhere to die, especially after Andrea and his last girlfriend...(he'll blame Walter for telling them about Andrea and for letting the other one die).
In any case, I love how none of us have any idea how this will play out and the big gun could be for more or less anyone. Pretty sure this is the best show ever on television... I thought Lydia would get the Ricin, but now it could be Gretchen & Elliot, or even Walter...
Looking forward the series finale.