Cruel, because it's like letting out our inner desires and frustration to a character. It is like projection and wish-fulfillment, no matter how shameful it is to admit it. Sometimes, we just really want to disembowel
punch that annoying person and assert our dominance, right?
We are obligated to be nice, acceptable, and moral all of time no matter what kind of frustration and inner struggles we have. Sometimes we want a break, want a catharsis, in an appropriate way that will not get you in trouble; writing cruel character is one of the way.
To write a good kind/cruel character is its own problem, though. It needs research, empathy, and social observation skill, which I cannot do the first and lack the latter two. It is hard.