You should be aware that removing someone from life support is not one of the four valid methods of execution. Note the story of the mekoshes aitzim who had to be stoned.
Secondly, only an official Sanhedrin can issue the death penalty and (Sanh. 41a; TJ, Sanh. 1:18a) the rabbis agreed that with the destruction of the Temple the Sanhedrin was precluded from inflicting capital punishment. In fact, the Sanhedrin could only impose the death penalty in the Lishkat HaGazit (Chamber of Hewn Stones). Only the full Sanhedrin of 71 could give the death penalty as the Sanhedrin of 23 could not do so.
As a result, the case that you specify would not be allowed to occur, even if the bais hamikdash had been rebuilt and the Sanhedrin re-instituted.
As a citation: The Sanhedrin and its importance to the Torah and Temple
Sanhedrin Ketanah (the Smaller Sanhedrin) sat in the Cheil outside the
eastern gate of the Ezrat Nashim (Court of the Women). This court had
23 judges and tried cases not requiring death penalties.