The only kindness worth its name is toward those who do not deserve it. To be kind to those who are good to us comes naturally, even reflexively, to all but sociopaths.
The secret to kindness in this sense is not to be found in the object. By definition, in the moment, they not deserve it. Rather, it is found in the subject. By taking our own nature into account, we realize the sense of indignation can just as easily have arisen from some other source, and the object may be nothing more than collateral damage.
I cannot recall a single, good decision made in anger.