"End Of A Young Gun"
When Will Fulton was recovering from a broken leg at the McCain ranch, he told Lucas how his parents had been killed when a man named Carson stole their farm.
If it's the man I'm thinking of, I don't think this Carson would have done the job himself. He probably had men working for him to do his dirty work. Everybody would know that they were Carson's hired thugs, but there would never be any proof to that fact. It would have been these outlaws who committed the murderous land grab, but it would have been on Carson's orders.
And who was this man named Carson?
I think he was the former Senator from Nevada, perhaps related (at least in Toobworld) to Kit Carson after whom Carson City, Nevada, was named. He was seen in the 'Bonanza' episode "The Iron Butterfly".
Carson was a bitter, brutal man by the time he ordered his men to kill the Fultons, having lost his son in 1871 during an altercation with Hoss Cartwright over an actress. Carson served as one of the state's senators at some point between 1864 (when the state was founded) to some time after the events of "The Iron Butterfly" took place. (Although it could be he was already retired from Congress by 1871; Ben Cartwright would still have addressed him as "Senator" as a formal courtesy.)
By 1881, when Will Fulton arrived at the McCain ranch, Senator Carson was apparently focusing his energies on building his personal wealth and his own private empire - by stealing the land of his neighbors.