Why blame for Jesus's death

As others have said, the fact that good came from Christ's death does not justify those who did it for evil reasons. This isn't specific to Christ's crucifixion. There are times when a crime or other evil action indirectly results in good.

For example, suppose a criminal murders someone. The victim had signed up to be an organ donor, and as a result of his death his organs are used to save the lives of several other people. A lot of good could come out of that person's death. Does that excuse the murderer from responsibility? If at his trial he brought in these people whose lives were saved, would the court find him innocent because of that? I certainly hope not. When he killed his victim, it was not his goal to help these people, he just wanted to get revenge on this person he hated or whatever his motive was.

Just because something is ordained/providential doesn't mean it's good.

God is able to use things even contrary to his nature to accomplish his will. Like a judo master using his opponent's force to his own advantage.

He used neighboring kingdoms to bring judgement upon Israel when they turned away from him and at some places calls even the pagan kings his servants.

Jeremiah 43:10 I will certainly bring my servant Nebuchadnezzar, king of Babylon, here to Egypt.

This doesn't mean God approved the practices of these kingdoms or saw them as "good".

Secondly let's consider for a moment that this was ordained/predestined. Then those who killed Christ didn't act by their own. If they didn't what is the credit? How can we avoid the blame by saying that they were forced to do it (by predestination/ordination) and yet demand credit by saying that we did it? That's doublethink. You can't have it both ways.

Bible specifically warns against this line of reasoning -

Romans 3:8 "(Some say)Let us do evil that good may result. Their condemnation is deserved."

You are right! dwelling on who killed Christ, was something even the early believers did not do, but rather why he offered up himself to die. Technically speaking the person to blame for Christ’s death is ourselves. ‘You’ are the reason he died, you must blame yourself, because it was for ‘your sin’ that he was punished. To put the focus away from ourselves is a great sin. Only when we understand that he died for my sin, do we become Christian.

Of course historically the Jewish authorities outwardly killed their own Messiah by making a false charge and pressuring the Romans to crucify him, but that is beside the point. God destroyed Jerusalem as a result of that crime, but they were not guiltier then us when looking at the real reason he died. To switch the focus of our sin, to something that happened so long ago seems to often indicate nothing more than wicked and racist thoughts.

By the way my wife is a Jew (converted to Jesus after we met).

At least as far as Judas is concerned, there is an exact answer in Matthew 26:24:

For the Son of Man must die, as the Scriptures declared long ago. But how terrible it will be for the one who betrays him. It would be far better for that man if he had never been born!"

I've also heard it phrased "but woe to him through whom it comes."

That said, God is sufficiently great to turn even evil events to his purpose. Joseph says as much in Genesis 50:20:

"As for you, you meant evil against me, but God meant it for good in order to bring about this present result, to preserve many people alive. (NASB)

Finally, Romans 8:28 (a verse that it should be illegal to quote without a license!) hints at the same, saying:

28 And we know that in all things God works for the good of those who love him, who have been called according to his purpose.

^{Disclaimer: I never have, nor never will tolerate any form of anti-Semitism based on "Christ-killer" propoganda. As a German student who won a trip to Germany for my study, I was personally sickened when, as part of a class, I had to watch a Nazi propaganda film, that pushed this line of thinking. The question asked for a justification. I am providing that argument, even if I completely disagree with it.}