Even though the mistakes that people make may play into a larger situation with good results, that doesn't justify their decisions or actions. It's a "the ends justifies the means" argument that doesn't wash. Judas was not forced to do what he did, but made free (and tragic) choices. Jesus tried repeatedly to reach out to him in friendship, but Judas would not be dissuaded. Even though Judas played an important role in the plot, that does (1) justify his actions, (2) mean that he was determined (forced) to do what he did, or (3) that he was as responsible for our salvation as much as Jesus was.

He's a bad buy because he betrayed a righteous person for money, helping to precipitate an illegitimate murderous act of an innocent man.

Surely if the point of Christ was to get crucified for our sins, Judas played an important part in that, and thus is as responsible for your salvation as Jesus was.