Everything bad in Kylo Ren's life is all Rey's fault. He's ten and his Uncle shows up with a newborn baby girl, notices wee Ben acting out with the Force around Leia's place, and suddenly Ben's kicked out of his home to go to Jedi Camp. Five years later, Baby Rey arrives at the same camp at the age of five to join the younglings, and all the instructors keep gushing about how strong she is in the Force while telling Ben he's not trying hard enough to control his anger and to go meditate on some trees and crap instead of taking part in fight training, so at 15... well. Things happened. Rey escaped the massacre and Kylo knew she would be back to screw his life up again. Then 9 years later she turns up, and his planet-sized base is destroyed. It's all Rey's fault.

Of course Snoke has been whispering in Ben's mind since birth, and the convenience of a baby cousin that keeps turning up in his life at convenient cusp moments? Snoke is not one to waste a scapegoat.