I am not sure that execution is exactly what everyone would call social justice, but it was the punishment that many people wanted and that has to suffice. All we know is that he acted alone in killing the 168 victims during the Oklahoma City bombing and execution has to suffice.
Timothy McVeigh very cruelly did bomb the Oklahoma City building leaving a lot of dead and a lot of victims. There will never be enough justice for a husband who lost his wife or a mother who lost her children. However, he is dead now so what else is there to do.
To start with social justice does not mean what the author of this question believes it does. Secondly, what more justice could there have been? Death was (and is) the ultimate form of punishment. What more could be done? We do not torture in the US, as is prohibited by the Constitution under "cruel and unusual punishment."
Timothy McVeigh's execution was a social justice. He was a murderer and an terrorist, and he deserved to pay the ultimate penalty. There was no greater punishment that could have been given to him. It was just to end his life in order to give closer to the surviving victims.
Without a doubt, the execution of Timothy McVeigh for the 168 victims of the Oklahoma City bombing wasn't anything near social justice. He got the easy death rather than having to suffer and rot in jail through an extended term. The fact of the matter is that death wasn't the best option here.