Very Far-fetched Conspiracy (Spoilers)
Killing 2 Supreme Court judges because they might rule against you on a case in a few years!! Any rich and powerful person is going to weigh up risks versus benefits. Being linked to multiple murders is a terrible risk to run. And the benefit is so uncertain. It is not possible to predict how a judge will vote in every single case.
And how did Darby come up with the Pelican Brief? OK, the 2 judges voted together on environmental issues. But how many environmental cases would there be across America that might eventually end up in the Supreme Court?