Hi,
I'd like to add a comment.
My dictionary defines kindness as 1. state or quality of being kind. 2. a kind act.
eg I believe that Confucius said 'Forget injuries, never forget kindnesses'.
eg There's an Irish saying 'May you have a thousand kindnesses'.
Having said the above, I do think that in certain cases where we are using meaning #1, we can say things like 'He has a cruel kindness in his character'. 'A' emphasizes the specificity and exceptionality of what we are focusing on.
Best wishes, Clive