[@Footman] A good idea, but [@Delta44] brings up an important point. More than half of the cast aren't really heroes. They're mercenaries. Maybe after the battle Izerk wagers some money for the help and the mercs hear him? We have to create a situation where even if you were just looking out for yourself, you'd intervene. 

EDIT: Maybe someone hired all the mercenary-type people to intervene already, and since those who aren't mercs are generally good, they'd probably hop in too?

EDITX2: In fact, I believe I've figured it out. A mysterious benefactor will have payed a few mercenaries (the not-good PCs) in advance to stop the executions. A few heroes (the good PCs) hop in. Afterwards, said mysterious benefactor then gathers our heroes and the adventure begins.