Prevent the creation of Magna Carta. I mentioned that at the beginning.
Essentially, he needed to have King John either so discredited that no one would even consider the idea, or. He could arrange to have King John killed even more easily before the document could be signed by his hand. This would, in turn, change the entire outcome of King John's reign and the eventual formation of the parliamentary government England has in later centuries!
Very subtle, really. Such a small thing, a lack of a signature for a monarch.
... much more subtle than the Master's usual fair...