What I think is ......
Stefan leaving with Klaus was not only for Damon but for Elena as well. Remember how he thought that the problem of Klaus' hybrids was because of Elena still being alive? And as a Ripper, once he started drinking those bloods from the last episode of Season 2 it triggered his ripper side. Blood is something he can never run away from.
How the writer made all our mind go crazy is just epic! All the next episodes will surely blow our minds. I totally agree with @Sean. Can't wait to see how Stefan's gonna take down Klaus. :)
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