Arthur, Guinevere, and Lancelot have all made grievous errors of judgment, so no one person out of the love triangle should shoulder the blame. It would be easy to say that Arthur should take the responsibility for the fall of the kingdom, since he is the king and should be the responsible one, but there were just so many factors working against him. Beyond the adulterous affair of Lancelot and Guinevere, Arthur also had to contend with Mordred trying to steal the throne. The end of Le Morte d'Arthur reads almost like a comedy of errors, because so many things go needlessly wrong.
So even though I am reluctant to place the blame on any of the three from the love triangle, I would like to blame Morgan Le Fay. She is a scheming, evil witch who has been trying to bring Arthur down from the beginning.