Well, I wasn't around in the Middle Ages, even if I'm an oldfag, kek …
That said, nowadays, as for who killed Christ, I believe thusly:
Humanity killed Christ.
Yes, that includes Romans.
Yes, that includes Jews.
Yes, that includes all sinners … past, present, and future.
For Catholics, we believe that Jesus' death on the Cross preserved His mother Mary from sin before she was born.
Christ came to save Humanity, and it was Humanity who killed Him.
So for anyone to single out and blame a certain group of people, well, that's basically un-Christian.
Thanks for letting me set the record straight.