I seem to recall someone arguing that, in combat between iron age tribes, it was basically a choice between massacre and slavery - if you did neither, they would wreck revenge upon your tribe further down the line.

This wouldn't be charity, as I guess the winners did benefit from having a source of labour that didn't need to be compensated at the market rate, but it would be a case where slavery was beneficial to the victims.

(I think Carlyle was wrong about other supposed cases of slavery proving beneficial for the victims)