Lincoln. Even after he won re-election he had many enemies in Washington.
May be no truth in it, but the theory has been advanced that Stanton was complicit in the assasination. If Johnson had been killed, and he was not only because the hit-man lost his nerve, with Seward badly wounded, Stanton could have made himself dictator. What he might have had in mind, we don't know. We do know he was enraged when Sherman cut a deal with Johnston.
With Sherman in favor of Lincoln's policy and with Grant bound to back Sherman's hand, nothing was going to happen.
Thanks for the historical info.