merciless

But to be fair, Slumdog Millionaire probably wasn't aiming for documentary mercilessness.

His effects are helped by the fact that he is an excellent mimic and by his utter realistic mercilessness.

He actually found his heated mind making a statement of the case as wild as this and its very mercilessness of phrase checked him.

Slavery has no respect for the affections, as is evinced by the mercilessness with which she sunders every family tie.