A common phrase, but has anyone ever literally signed their own death warrant ?
Michael Collins, in a manner of speaking. But still only in a figurative sense, so I don’t know if that’s close enough.
It’s not quite a death warrant, but close: French revolutionary Robespierre was tried and condemned under a law according to which the evidence requirements to condemn persons to death for anti-revolutionary activities were significantly lowered. Robespierre himself had supported this law, and its enactment was one of the reasons for his eventual downfall and execution. Because of this, Robespierre has been said to have figuratively died by the hands of his child.
Since signing a death warrant involves a person having the authority to condemn people to death, it is hard to imagine a situation where such a person would “officially” condemn HIMSELF to death. Another person with similar authority (but presumably lower in rank), maybe, but himself? (And not due to clerical error, quickly set right?)
The closest thing I can think of would be ritual suicide for political crimes, like “obligatory seppuku” in feudal Japan. For example, a Westerner described witnessing the hara-kiri of one Taki Zenzaburo, who had ordered an attack on a settlement of Westerners in 1868. As part of the smoothing over of the incident, he was required to apologize in person to representatives from the governments of the nations so attacked, admit full responsibility, and then kill himself.