After witnessing these Bandits kill a small pack of Orc Young, I attacked them before they could move or run off into a forest, only to be put in the corner and wait 6 turns for them to run to me.
Unfortunately, that’s always what happens when you join an in-progress battle, regardless of whether one of the original sides has been wiped out already. You also always start in the bottom-left corner when you enter an old abandoned site with unknown inhabitants, or attack a graveyard (for a contract) with inhabitants that are not revealed until you enter.