[Commonmind]

It's a safe assumption that he could also be referring to the impending duel between Harry and Voldermort, and not necessarily Harry's death. If there was a duel written in the heavens, it's likely Bane would have been upset by Firenze's interference; for two reasons: it could mean Harry is destined to die, or it could mean he's destined to destroy Voldermort. Despite Bane's aggressive manner, I'm sure he also wants Voldermort dead, and Firenze getting in the way of the duel that would bring about that outcome would be an upsetting turn of events.