The Bible teaches that Jesus died according to God's plan to provide salvation for man. So some suggest that it was "nice" of the Jews/Romans to kill Jesus because otherwise we would not have salvation. Even if they were not nice, they were at least innocent because they were only carrying out God's plan.
But this is not so. The Bible affirms not only the sovereignty of God, but also the free will of man. In fact, the very passage that tells us that Jesus died according to God's plan (Acts 2:23-24) also implicates those who crucified him. God did not "program" the Jews and Romans to crucify Jesus. He merely knew their attitude in advance and so allowed Jesus to die at their hands. The Jews made a conscious choice to crucify the Lord Jesus (Matthew 27:23-25). Pontius Pilate made a fully conscious choice to crucify him (John 19:12-16) and the Roman soldiers (representing all of us) made a conscious choice to unnecessarily increase Jesus' agony (Mark 15:16).
Besides, a three-hour ordeal at the hands of Roman soldiers is not enough to atone for the sin of billions of humans. Jesus suffered at God's hands on the cross (Isaiah 53:10), and it is this that makes salvation available to us today.
Thus, the people who crucified Jesus are guilty, and we have no obligations towards them. At the same time, it is absolutely wrong to demonize modern Jews as "Christ killers".
It would be, if there was compulsion involved or the sentence was transferred. If you jumped a signal, and the traffic police conveniently ignored you, stopped me and fined me, that would be unfair. But if he fined you, and you didn't have the money, I could come over to you and give you the money to pay the fine. In fact, when you pay a fine, the cop never asks you whether you've earned the money yourself, or whether someone else gave it to you. It is fair enough if you are sentenced, and the fine money is paid.
In the same way, we are sinners, and God sentences us - not Jesus Christ, who is sinless. But the Lord Jesus voluntarily pays on our behalf what we need to pay. So it is not unfair - just amazingly kind.