I'm going to venture a guess, because I'm not sure about the laws.
Jhn 18:28 Then led they Jesus from Caiaphas unto
the hall of judgment: and it was early; and
they themselves went not into the judgment hall, lest they should be defiled; but that they might eat the passover.
Jhn 18:29 Pilate then went out unto them, and said, What accusation bring ye against this man?
Jhn 18:30 They answered and said unto him, If he were not a malefactor, we would not have delivered him up unto thee.
Jhn 18:31 Then said Pilate unto them,
Take ye him, and judge him according to your law. The Jews therefore said unto him, It is not lawful for us to put any man to death:
Jhn 18:32 That the saying of Jesus might be fulfilled, which he spake, signifying what death he should die.
Jhn 18:33 Then Pilate entered
into the judgment hall again, and called Jesus, and said unto him, Art thou the King of the Jews?
So what I'm thinking, and any help with this would be appreciated, is that because it was the morning of the passover, they could not judge and execute on that day? without defiling themselves and missing out on the passover meal.
So they accused him of being a "malefactor" not sure what that is but I'm thinking a thief.
Because they claimed that Jesus was basically trying to steal Caesars kingdom from him, by claiming himself a king?
So they covered their... by putting the judgment and execution in the hands of the Romans.
When it comes to Stephen, it was just an ordinary day. Not a sabbath or holy day, so they were not under a law that pevented them from stoning.
Just my thoughts.
Hugs