Gotta love blackmail
An unfortunate fact of life is that not everyone is willing to keep their promises, especially when it's only verbal.
More often or not, one member of a verbal promise will attempt to renege on it if it means they can get more benefit from doing so, or at least protect their "reputation".
It's rather tricky to have people believe you are in the right, when everyone is relying on he said, she said.
This is where contracts come in.
Solid, physical proof that a deal took place, notarized documents that make it clear to the world that both parties made a promise to each other to perform the depicted task or deal to its letter.
Fine pieces of evidence to prove whether or not someone is being a lying bastard.
Or in this case, prove that someone orchestrated the death of their own mother.