Some believe that God predestines all things.
Yes. This is a Transcendent level understanding of God’s metaphysical relationship to “all things” created and sustained by him at all moments of the timeline of creation.
If that is so who killed Abel?
But here you ask a Storyline Level question.
Answer: Cain killed Able
It’s like stating J.R.R. Tolkien “predestines all things” that occurs in his creation The Lord Of The Rings…
… then asking “If that is so who killed…” Gandalf?
This is a Storyline level question?
Answer: Balrog killed Gandalf
Does this answer change the fact that Tolkien predetermined when where and how Balrog would kill Gandalf in the timeline of his creation?
Answer: No because both are true on different levels.