Jesus is a walking oracle. He's always tossing out prophecies, many of which actually come true in the course of the narrative. We're probably supposed to think that several others will be fulfilled beyond the story's end in 16:8, too (for example, 13:1-37 and 14:28). In some cases, these prophetic words have their source in the scriptures (9:12; 14:21, 27), which are read as authoritative guides for the future (1:2-3).
What's the long and short of it? Fate, where God is the silent mover. Certain things "must" (an important word in Mark) happen, but where exactly humans are free to will their own actions is an open question. Does Jesus imply, for example, that he could choose otherwise in 14:36?
One thing is for certain: humans are responsible for their actions even when they are fated. Sorry, Judas.