Which preposition to use with unmerciful

to Occurrences 3%

The South is willing that the Tract Society should expend its money to convince the slave that he has a soul to be saved so far as he is obedient to his master, but not to persuade the master that he has a soul to undergo a very different process so far as he is unmerciful to his slave.

in Occurrences 2%

If this man were unmerciful in his revenge, he also remembered my mother's kindnesses, and bled for her and her daughters.

Which preposition to use with  unmerciful