The Talmud says in Tractate Makkot:
A
Sanhedrin that effects an execution
once in seven years is branded a destructive tribunal. Rabbi
Eliezer ben Azariah says: Once in seventy years. Rabbi Tarfon and Rabbi
Akiva say: Were we members of a Sanhedrin, no person would ever be put to
death. [Thereupon] Rabban Simeon ben Gamaliel remarked, [yea] and they would
also multiply shedders of blood in Israel! [Makkot 7a]
All executions were, by Roman law,
to be carried out by the empire.
Whether it was the extreme brutality of
the Romans or some disagreement between members of the court, something happened in 30CE that
affected the Sanhedrin (the supreme court of first century Judaism) so much
they physically removed themselves from the Temple so they could never again
impose the death sentence. And indeed the death sentence was never again
imposed.
Again, from the Talmud in Tractate
Avodah Zara it says:
Forty years before the Temple was destroyed
did the Sanhedrin abandon [the Temple] and held its sittings in Hanuth. Has
this any legal bearing?... Capital cases ceased. Why? -- Because when
the Sanhedrin saw that murderers were so prevalent that they could not be
properly dealt with judicially, they said: Rather let us be exiled from
place to place than pronounce them guilty [of capital offences] for it is
written: And you shall do according to the sentence, which they of that
place which the Lord shall choose shall tell you, which implies that it is
the place that matters. [Avodah Zara 8b]
That, however, was not the only change in
the Temple at that time.