Yes I think he blamed SW....and I think he checked out of his relationship with his daughters too, and didn't want a relationship with his son (or didn't want to face of a potential failed relationship with his son, now that I think about it).
In the doc dump, I noticed he claimed SW criticized him to the girls and they were starting to repeat what she said about him. I do NOT think the girls turned against their father, but that CW was hyper-sensitive about finding fault in SW. Kids say annoying things sometimes, and it's my guess he was angered by the girls and blamed SW.
So...it's my hunch that the girls acted up in their comments and I definitely think they were a handful for him to handle while SW was in AZ....and understandably so! Even if the best of circumstances, coming home from weeks out of town with tension in the family, only to have mom turn around on a trip for the weekend would be hard in ANY family. Traveling and transitions can be hard on little ones. Add on a birthday party that likely wound the girls up.
I think dinner that final night was a complete disaster and it was the straw that broke the camel's back. And it was SW's fault for not being there and it was her fault that when she was there, she was too controlling (in his mind).
When dinner didn't go smoothly, he lost it. He took out his rage on the girls and then finalized it all by taking out his rage on his wife and unborn son. He might have been planning the murders for awhile, I don't know, but I think the murders happening on that particular night were a mix of planning and impulse.
jmo