Ye have heard the blasphemy: what think ye? And they all condemned him to be guilty of death.
Note 10 at Mr 14:64: As mentioned in note 6 at Mr 14:53, it is probable that Nicodemus was present at this mock trial. Mark said that "they all condemned him to be guilty of death." If Nicodemus was present, that would mean that he participated in this sentencing. Yet we find that Nicodemus brought sweet spices that were worth thousands of dollars to anoint the body of Jesus for burial this same day, implying that he certainly felt differently about who Jesus was than the rest of the Sanhedrin.
It is possible that Mark's use of the word "all" in Mr 14:53 and 55 was similar to the way he used it in Mr 1:5 (see note 6 at that verse) or that Nicodemus was not present at this trial.
It is also possible that Nicodemus denied what he felt in his heart and voted with the rest of the Sanhedrin out of fear of persecution even as Peter denied Jesus during the same time.
Maybe Nicodemus repented as he witnessed the crucifixion and saw Jesus' love for the very ones who killed Him. There is no way to know for sure what really happened to Nicodemus with the limited information that we have.