It seems that Yeshua did commit treason according to the laws of Rome:
"When asked by Pilate whether he was the king of the Jews, Jesus replied, 'Thou sayest it', thereby virtually pleading guilty to revolting against the Roman emperor and the king recognized by him...There can be no doubt that a confession such as this was sufficient in Roman law for conviction of the defendant. Nor can there be any doubt that it was this charge of claiming to be the king of the Jews that was the ground for conviction and sentence; proof is furnished by the fact, reported by all the Gospels, that the words Rex Judaeorum were inscribed on the Cross, and the inscription on an offender's cross was prescribed by Roman law.
The (armed or unarmed) insurrection inherent in the claim to be king, without being appointed or recognized as such by the Emperor, was an offence against the Lex Julia Majestatis, enacted by Augustus in the year 8 B.C. The offence was punishable with death," pg. 88 of Jewish Law in Ancient and Modern Israel by Haim H. Cohn
It's also worth noting that the Sanhedrin lacked the authority to try capital cases:
"The fact that Jesus was tried by Pilate, presumably under Roman law, does not exclude the possibility that he had previously (namely, the night before) been tried by a Jewish court, presumably under Jewish law. This sequence of events is that prima facie suggested in the Synoptic Gospels (Mark, Matthew and Luke--except that Luke places the Jewish trial in the early hours of the morning). The theory that the only 'real' trial took place before the Jewish court, and that Jesus was delivered unto Pilate for purposes of execution only, is now generally discarded. It is also incompatible with the tradition of the Gospel of John, according to which the trial before Pilate was the only trial and that what preceded it was but an interrogation of Jesus by the HIgh Priest. According to John, Pilate called upon the Jews to take Jesus and try him themselves and they replied that they could not lawfully try capital cases. If that reply were correct, it follows that either no Jewish trial did in fact take place, or that it was a fact-finding trial only, without competence to pass sentence," pgs 85-86, ibid.
"....The Forty Years tradition is found in the Talmudic sources in two version: one (the Jerusalemite) speaks of the cessation of captial jurisdiction ('forty years before the destruction of the Temple, the capital jurisdiction ('forty years before the destruction of the Temple, the capital laws were taken away from Israel.' Y. Sanhedrin 7,1); the other (the Babylonian) speaks of the banishment, forty years before the destruction of the Temple, of the Sanhedrin from its hall of justice in the Temple precincts to a 'shop', where according to the law no Sanhedrial jurisdiction could be exercised (see Deuteronomy 17, 8-10)," (footnote 15, pg. 86)
I guess Peter meant that the Israelites were responsible for instigating the trial and for influencing the proceedings.
But, of course, Yeshua died for us all. So we're all responsible.