"Pass" can have two meanings here. The first would be "to encounter in passing", as in each one was going in a direction opposite to the other when they encountered one another. The second would be "passing" digestive matter, e.g. a bowel movement. Cannibals eat other human beings, so the wordplay here is that he is presumably excreting the digested remains of his brother.
The cannibal passed his brother in the woods .