iserith
Magic Wordsmith
If I were to make that same mistake, I would handle it by not changing anything except being sure that I convey to the players and their characters that they are facing overwhelming odds - letting the players work out their own solution - and by having the brigands consider not killing a priority because the punishment for murder is more severe than the punishment for robbery, hostages might mean ransoms, and any other way to naturally change the failure result of the encounter to something besides "everyone died."
Yep. Telegraphing and stake-setting is an easy fix here.