As mentioned above, in TFA Leia told Han that Snoke had been manipulating Ben Solo since childhood. Then in the Last Jedi we found out that the final straw was him waking up to Luke about to kill him.

I don't think they need to explain more than that in the films, but in the books they do explain even more. In Bloodline it goes into more detail about Snoke's manipulation - Leia could sense Snoke manipulating her son even while she was pregnant with him. So it sounds like it was a long term manipulation similar to Palpatine and Anakin Skywalker.

Also in Bloodline it talks about the galaxy finding out that Leia was Vader's daughter - this was something kept secret for years, and Ben Solo himself didn't find out until he was in his 20s when that info was leaked. He was pissed at his family for hiding that info from him.