Skyler is the only thing standing between Walter and the thing he wants: his children. At least, that's what he tells others, and himself, that's what he wants. I also feel like Walter has nearly no love for Skyler anymore, and thus, would be open to the possibility of killing her if he felt like it was the only way he was going to be with his children again.
We've seen Walter go from being torn up inside at the idea of having to kill someone (Krazy-8) to murdering a plethora of inmates in the prison without any hesitation. Now, we need to see him do something we haven't seen him do before. Something that brings him even further down this dark path. Killing his wife, one of the people he got into the drug business for in the first place, would take him there.
Because of all of this, I thought throughout season 5 that Walter would kill Skyler eventually, even before I saw all of the evidence about Walter taking on character traits of his victims.
And there's a little "rule" you hear people say a lot in playwriting (that applies to all types of fictional writing): "If you're going to put a gun on the wall, it has to go off by the end of the play." For the entire series there has been "a gun on the wall": the ricen. The writers have been dangling it in front of us for as long as I can remember. It would seem strange if the ricin was never used on anyone. We know Walter still has it. I believe he's going to use it. On Skyler. After he does, Jessie might start to get suspicious about Sklyer's death and how similar it sounds to the way Walter originally told him the ricen would kill someone. How she was fine, then all the sudden just dropped dead one day.
It would create a lot of interesting drama between characters. Now Hank has very good reason to believe Walter is Heisenberg. Jessie is suspicious of Walter killing Skyler with the ricen. Even Hank will be suspicious about Skyler's sudden death with the knowledge that Walter is a drug lord.
I don't know how it all resolves, but I think the killing of Skyler is what will set off the bomb of the remaining events of the show.