Since when has the institution of slavery been a charitable one? Historically, slave-owners have payed immense costs, directly and indirectly, for the privilege of owning slaves, and done so knowingly and willingly. It is human nature to derive pleasure from holding power over others.
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)
Here's the new thread for posting quotes, with the usual rules: