Really? I haven't played since plots were added. If you can reliably kill 2 rival kings within 2 years then it's no wonder people are saying the game is too easy. That's ridiculously overpowered. Regardless though the fact remains it wasn't marrying Matilda that broke the OP's game.
Maybe you're right. But it was actually easy to kill the kings of Galicia and Castille. I started as Leon with Alfonso VI. Historically, he killed his brother Sancho to win Castille because he lost war against him (El Cid Campeador truly helped Sancho on this). To represent this in the game, Alfonso VI has a score of 24 in intrigue, which is huge. It's not difficult to bribe some Castille and Galicia courtiers to make them join the plot, and if everything goes well, it's quick.
The whole luck for me was marriages and dynasties issues. First Matilda. She gave me a heir before dying fighting the Kaiser. My son Fernando was duke of Tuscany but I married him to the duchess of Aquitaine (a girl with no heirs) and she died before me so my son inherited Aquitaine who was now inside Leon as you can see.
Then, a big dynasty issue (which I wasn't responsible of as I was busy fighting the Reconquista) occured in France. No heirs left. Big luck, the guy who took the throne was Waldensian. So France was a cake, and almost everyone in Europe took a piece of it during holy wars(even Navarra took a part). I took Barcelona, and my clever son, duke of Aquitaine attacked to take Toulouse.
Five years later, the Pope called a crusade over France. I won it pretty quickly, the HRE didn't join. And Hispania was born.
So you see. Big loads of pure luck. And it all started with Matilda. Nice gal.