Cruelty is the act of being cruel. That is to say, needlessly and excessively unpleasant. Torture is cruel. Waving Bleeprin at agents who've returned from smutfic and then not giving them any is cruel. Volunteering your partner to work in continua inhabited by their Lust Objects is cruel.
The PPC are against cruelty to canon characters, but may push the envelope in the name of poetic justice when it comes to Mary Sues. Torture is right out, though.