Was it not the Sadducees that were mostly responsible for Christ's death? They were significant as part of the ruling Sanhedrin authority at the time and the major influence in the Sanhedrin. Caiaphas, the chief priest of the Sanhedrin was a Sadduccee.
"The polemical utterances in the Gospels...created the image of the Pharisees as religious hypocrites--an image from which only modern research has freed them." Encylopaedia Brittanica. So who do we believe in this matter?
As for who is "mostly responsible for Christ's death", I suppose it depends on your understanding of Christian theology. Christ has some harsh words for Peter when he tries to prevent it. Paul says we would all be most unfortunate if it hadn't occurred. Christ's death is necessary, so I don't know if blame is really an appropriate consideration.