One of Luke's major concerns is to show that the work, passion, crucifixion, and resurrection of Jesus are the fulfillment of the Jewish scriptures (i.e., Moses, the prophets, and the Psalms). This is Luke's main way of coming to terms with the fact that Jesus—the Messiah, for crying out loud—gets crucified. God's will has been accomplished precisely through and in spite of all of those acts of opposition, betrayal, and rejection.
It's a pretty hefty concept of divine fate, necessity, determination, or even destiny. And the pesky conundrum arises: are the people who betrayed, rejected, and condemned Jesus responsible for their actions? One thing's certain. Within Luke's text they are held responsible and their punishment is expected (Judas is a case in point; see 22:22). But it's a tough line. Work through the texts for yourself and see if you can toe it.